So sad that you and you kids are dealing with this. 

The silver lining is that now you can be 100% sure you made the right decision. 

My SILs SO had this happen to him. He’d been gone a week and when he went pick his kids up another man, shirtless, wearing pajama pants no less! answered the door. He just shrugged and said good luck buddy. 

Then about a month later the ex called begging him to come back. There was nothing between her and the guy. She was just trying to make him jealous! So so sad.

I understand you are upset.  It does take awhile to get over this betrayal.  My EX left one day and moved in with his girlfriend (now wife) the next day.  It was very painful.  They have been together for about 16 years now.  I never thought they would last more than a few weeks.  My kids have told me manystories over the years and I realized that even though they may still be married, they do not have the happy ever after.  It was very hard in the beginning to see them together.  It took me a few years before I could deal with it without getting upset. I never thought I would ever get over the betrayal.  Now, I don't like either one of them but I am cordial whenever they are around.  In my opinion now,  they were two very selfish people who really deserve each other.  She was married too.

You are so much stronger than you know. You are going to be fine.  You are going to find a better life.  Just breathe.  It will get easier, I promise.

If I remember correctly, and I might not because I can’t keep track of stories and usernames so apologies if I’m wrong, your now Ex was the guy who had the Stepdad who would pay for the BM and skid to have new things like Boy Scouts. 

To you I say: let it go. If his stepdad interfered with y’all’s finances and skid relationship like before then he will run this new girl off in no time. I’m sure they’ve received her with open-arms because she’s new and not you, but a crappy dad is a crappy dad no matter who he’a with or got to cover his slack. 

Whether it be a few months or a few years, divorce is never easy for kids. 

I especially hate it when a divorcing/divorced male or female moves in their new GF/SO/DW/DH into what was the 'family home'. I believe new relationships deserve a fresh new start, including the home. Moving a new person into an established environment (aka current home) sets the new arrival as the outsider. Not a fair thing to do to the new GF/SO/DW/DH nor any children they may be bringing into the home with them. 

I'm sure it was indeed a shock to your girls to go visit where they've called home for a number of years, only to find they've been displaced. I actually find it quite disrespectful and inappropriate of your STB-ex not to have mentioned when he called, 'oh, by the way, just so you know, if the girls come sty the night , that ______ (lady's name) and her kids have moved in and will also be here'. 

It would have certainly given you the final decision on whether or not you thought the girls should go and stay or to have prepared them for what they were to find. That is one advantage you do have or a divorced BM where the children belong to the opposite parent. They don't get a choice to send them still or not, they have to send them... but even in that situation the mother/father have a change to prepare the children.

Children get the short end of the stick in relationships that are ending and new relationships are beginning.  I'm sorry your girls went with no clue and no preparation for what they would find. I'm sure they were hurt. I do hope though, that as long as they were invited and went that the GF and her children along with your SBT-ex tried to make the adjustment comfortable for the girls and welcomed them and made them feel included in the home's evening. I would hope that your STB-ex, their still stepfather, took the time to pull them aside and talk to them and concern for the girls surprise and confusion. If he wasn't going to show understanding and compassion to these two displace little girls, why invite them over e could have just taken the two girls out for ice cream along with just him and talked with him. I wish he would have. It's pretty cold to just drop such a zinger on children with no warning nor preparation. 

Get your divorce, OP. Forward march. There is a whole new life and happiness out there for you and your daughters. Perhaps your girls would benefit from some counseling as you are doing yourself? It's natural to grieve and the girls are grieving the loss of what was and what will no longer be. 

The man has moved on with his life.  You and your kids need to do the same thing.

I'm surprised his girlfriend allowed the sleepover.  That seems like a huge  boundary issue to me.  It gives the kid(and you) false hope.

She's, saying that now, but it won't last.

She's going  to get sick and tired of having 2 extra kids around.  As their family grows, there will not be room for kids who are not related (by blood or legally) to that man.

You are doing your kids a disservice by allowing them to cling to your exhusband.

Stb-exDH should have given everyone a "heads-up" on relevant changes before inviting the girls for a sleepover. 

It torques me off that his actions deliberately caused harm to children he knew.  Deliberately, willfully, consciously caused hurt for your girls.  Feigned ignorance is merely a diversion to hide malicious intent.

I don't care if you had a menage-a-trois with a donkey, nothing that you did in your marriage between 2 adults justifies a trusted adult knowingly hurting a child.

It is the OP's responsibility to protect her children, not her ex's.  Agreeing to the sleepover was a poor decision on her part. 

Blocking his number from their phones and making it clear that they are not to call him should be the next order of business.  He's not their father.  Expecting them to have a place in his new life is ridiculous.

Is the house still legally in your name? Go in one day when they are gone and paint a wall lime green. Rearrange the kitchen. Put a glamour shot photo of you up. 

4 months and someone new living in your marital home. Have you done a separation agreement? I would talk to your lawyer. I know you don't have kids together but you might still be able to get him for abandonment and adultry. I realize you said that you think they met after your split (so together even less than 4 months and moving kids and themselves in together- you can't see my giant eye roll, but it was there), but if you don't have paperwork signed you might have a case. Especially if you contest this divorce. Just thowing that out there. Then he can have you to battle and his babymomma drama. It will be too much for the new broad to handle.