BM here is absolutely stuck in time. SO and BM separated in 2011 divorced in 2012. I've been with him since 2015.

BM is remarried been with him probably 6/7 years now. (Don't know actually date) 

Her go to with skids is sending "time hops" of when they were small. She does this often. When I first started dating SO she had a group text with the boys and him and would send them to all 3 of them. About probably 6 months in i was like uhhhh this isn't ok. He talked to her about that and it stopped. She still continues to send skids them. Which is fine but it's just her way (I think) of her (in her head) thinking she's a "good mom." They have to be repeats now throughout the years. I'm pretty sure the ones she sends though also included SO in them. 
 

SD13s birthday in April she sent her a pic of her ultrasound of her. Really throwing it back haha. 
 

About a month ago she tried sending pics to SO of skids and he told her to send to skids and she got pissed.

Shes stuck in a time where she actually saw them. It's what she holds on too. 

The Skids may be catered to at the opposition  parent's home but they do not have to be catered to in our homes with their other parent.  

We adopted the immediate compliance model for SS when he returned home from SpermLand visitation.  As a toddler and little boy he would leave fully conversant, mostly toilet trained, and very engaging and involved.  He would come back in a loaded and soaked diaper, non conversant and tantrum throwing at any little thing. We would have a stern face to face with him. After that he complied or he experienced consequences.  He was fully capable of asking for what he wanted but would point, grunt, scream, and pitch a fit.  So, we just stepped around him while telling him to use his words or he would not get what he wanted.  If he chose to continue the toxic crap from SpermLand visitation he might get a swat to the butt and be marched to his room or a isolated corner.  Lather, rinse, repeat until he stopped the crap.

As he grew up, we adjusted. But never did we tolerate any previsitation behavioral degredation or extended behavioral crap when he returned from SpermLand visitation.  He struggled with previsitation behavioral degradation and post visitation detox at some level for the entire 16+ years we lived under the CO though we all figured out how to get through it quickly and increasingly effectively over the years.

This is definitely a thing. At first my DH always treated SD like she was forever 3 years old. I had to have a talk with him about realistic expectations for her. BM both babies SD and treats her like a peer. SD has lot of irrational fears that BM has encouraged and it's kind of sick. She wants SD to be dependent on her. 

Honestly i don't know if it even has to have much to BM. Sd is 15 yet she literally has about the same maturity level as my 7yo autistic neice.  She constantly cries over everything.  We were looking for stuff to get DH for father's day.  Dds15 and 16 got him a shirt that says something about being an awesome step dad.  Sd wanted to get him a pocket knife.  So i was trying to help her find one, them she saw this hammer that turns into a wrench, or screwdriver.  I told her theat he doesn't do a whole lot of handyman stuff, but he does love pocket knives, I was also suggesting travelers mugs, and and showing how they had baby Yoda on the  mugs (that's something they both like). She started sobbing and stopped even looking for stuff, i obviously hurt her feelings, i felt bad, so i told her we could get that hammer thing, maybe i was wrong and dh would love it.  It's up to her what she wanted to get DH. I just didn't want it to hurt her feelings if it gets pt in his tool box and never used.  

Then i was asking if she wanted to  wrap it separately, or make a big box of presents for them all to give dh. She started crying again,  saying how obviously DH would know that the shirt was from my DDs and then her gift would be "just there" and he might not know that she got him that. I was like "girl, if you want it to be separate, just say so." This girl! She still thinks boys are gross, at 15. DH bought her a night stand at a garage sale that was still in a box.... we opened it up and it was a bunch of random wire shelving.... she cried. I mentioned she seems to get abused reflux a lot,  she cries,  this girl! She's 15! She literally acts like this AAAALLLLL the time. She couldn't figure out how to user the credit card machine in check out, she cried! 

I joke that she is "forever locked in time". Shes now emotionally about 12-13ish.

She might have matured just because of her new drug use and promiscuity. That tends to age you. Ive been in her life since she was 15.5. When she lived parttime with us, she was extremely feral, stealing food and eating in her room at all hours of the night. Trashed her room. Grunted and yelled, temper tantrums.

Now she lives with her very stunted mother. And from Munchkin's reports shes the same and worse. Because she is now in party mode.

It should be noted that Feral Forger has not had a job in over a year, and at 21 no drivers license, hence no car (shes mad at DH because at her high school graduation she did not recieve her promised car.) The deal was that she would get a car when she had a job AND a license.

I think the lack of driving independently of people, and not having a job has stunted her growth significantly.

This is a very interesting way to look at things. For a very long time my SD was fixated on her parents being back together and still seems to haven't moved on. My husband and BM were teen parents and split a week before her 1st birthday. I have been with my husband since SD was 3 and BM got married when SD was 4. SD has never known life with them together but she couldn't move on from this idea. BM wasn't helping by telling SD that if her dad really loved her he would be living with her everyday. BM and step dad and my husband and I each went on to have 4 more children. So things have moved on but they still haven't. Despite being married now for several years with more children BM is still hung up on a split that happened over 13 years ago and will viciously lash out on my husband at random. SD takes it out on my husband and became very upset when they refused to have a vacation she was wanting that only included BM, her siblings from her mom, my husband, and of course SD. SD has also cut off contact with my husband because she only wants one family and he is not truly her dad. So there is still some hang up and despite therapy and acknowledgement of her feelings she absolutely can not move on.

Trauma can stunt psychological growth.  My SIL list his mother to cancer at 13. It really messed him up and he ended up in therapy to "grow up".  
The refusal to accept reality causes so many issues.  It hinders the development of resilience along with other life skills.  So many CODs can't function.  

When I first became a SM I read a lot about the impacts of divorce on children and there are some schools of thought that claim that a child's development is arrested at the time the divorce happens. SSs were 6 when DH moved out. When I met him, they were 9 and they definitely acted younger than they were. They couldn't (or wouldn't) sleep alone. They couldn't cut up their own food. They had to be reminded to do things like flush the toilet. They would cry and have temper tantrums. I don't even know if DH even noticed. 

Now, SSs are 14 and DH will say to them that he thinks they are regressing because they don't act their age, but really, I think they never moved forward that much, DH is just now noticing because he no longer feels as guilty about the divorce. 

I think this is probably more common when parents didn't really know how to parent before the divorce. For example, BM was never much of a parent. When SSs were small, she would get overwhelmed and leave the room, leaving DH to manage everything. She also can't deal if either kid is unhappy, so will do nearly everything to remove anything that is uncomfortable for them. The other side is that she leaves them home alone a lot to fend for themselves and has they are 9 or 10. Some kids thrive in that environment and are able to parent themselves, but many flounder and never move to an age-appropriate development stage. 

Traumatic events (and drugs) can stunt emotional growth and development. However, as an adult, BM should be able to move on and, as a parent, help her children move on. Although, the dads also contribute to their kids' lack of emotional maturity through guilty parenting.

BM over here, finally moved on but, of course, has always been the perpetual victim. My SD is very immature due to BM wanting her to be dependent on her, which I think is more due to BM's BPD than the divorce. Although, DH not being there everyday was traumatic for SD, as he was the main nurturer.