he "behaves badly with his mother because that's where he feels free"

Yeah, because that makes sense ... DUH

Do you think that's her own brain she's using or is she breaking it in for an idiot?

Unfortunately.. while it seems to defy logic.. but quite often.. it's not that the 'best" home is chosen.. that the one that can offer the most resources.. the priority is to have both parents in the child's life.. (often skewed to the mother).. so the best he may be able to hope for is that his home is deemed safe and that his custody continues.. not that they will necessarily "see" the dysfunction at mom's and take it from her.

Nanny cams in your home going forward, also never be alone with either of the skids, this way they can’t ever make anything up about you.  We had CPS come visit us after a court hearing where Spawn admitted to certain things about her mother.  They came to our house and looked around and wanted to see when Spawn’s room was…other than that their only remark was how clean our house was.  It was intrusive and annoying, but in the end it just validated to the courts that we were a good stable home.

During our court battle with BM, the judge told BM essentially to mind her own business unless there were concerns of abuse and neglect in our home. Well BM didn't like being told to butt out so, cue the abuse and neglect allegations, right. 

Our BM also made similar claims that SS10 was only comfortable being his true self at her house, which was a stressed out, emotional, scared kid. She said that he acted that way out of trauma due to the visitation he "endures" with DH and I, but that he wasn't comfortable sharing those feelings here. Yet, at our house, SS is mostly well adjusted, happy, and calm. 

BM made allegations of physical abuse after that court hearing and after a visit. She claimed that DH beat the crap out of SS with a NERF gun and when SS told DH to stop, DH continued. BM tried to claim that SS then locked himself in his room to seek refuge from the abuse. BM tried to tell DH that in their house "No means No, and Stop means Stop" and that we need to learn that and respect SS' boundaries. BM never took these allegations to CPS, the police, or court. We consulted our attorney and wrote a response back, refuting all the allegations, and asking for a sit-down with both parties and SS to discuss lying and false allegations. BM refused because we know she made it all up and didn't want to get caught, but she said she didn't want SS to feel ganged up since she is his "safe space." In response, we had a sit down with SS alone (video recorded for proof) and talked through everything. He knew nothing about the allegations - had a total deer in headlights look. We told him that all NERF guns were going into storage until he felt safe with us. He said that was never an issue. We knew BM was just using the email allegations to build a paper trail for court. 

After that, we installed cameras. One in every main common area and one in the hallway above SS' bedroom door. Our attorney advised us to put one above SS' bedroom door to record who goes in and out of SS' room in case BM tried claiming sexual abuse in the night, since you can't put a camera in a bedroom. After BM found out that cameras were installed, there were no further allegations because she knew she would never have proof and that anything captured of SS actually enjoying his visits with us could hurt her case. 

Your lawyer is probably saving you a lot of money on a fight she knows you won't win.

Alllegations like this just keep coming even if this one is cleared. Your SS wants the life of ease, no rules and no expectations. 

Your best bet is honestly to get cleared by CPS, then let SS go live with his mother. Take every other weekend visitation and call it a day. If SD wants to stay with you, let her- at least she will have a shot at having a decent life with you guys around. 

What is best for SS doesn't matter when it comes to courts and CPS. They will side with BM and your life will just be hell. 

I agree with the poster who said nanny cams are important from here on out. Keep them in all the shared living spaces. 

Madam, I have been in your position, only for 23 years and with  3 skids.  All I can say, is if BM continues parenting like this it doesn't end well.  Everything you mention in your post, I have experienced.  The current situation with the skids is as follows: SD32-single parent to a 2 year old, not working, living in a condo owned by us paying minimal rent, diagnosed bipolar; SS29, living with BM, not working for last 5 years, diagnosed borderline; SD28-in university for past 9 years, no degree yet, working as a waitress, most pleasant one of the bunch.  So congrats to BM, she successfully raised those children, just didn't manage to raise adults. Apparently adulting is just too hard for my skids.

oh, also I agree with other posters. Don't waste time or money on lawyers.  Unless there is provable abuse or neglect then children are generally not taken from their mother. And these BM's spend an inordinate amount of effort making them seem like mothers-of-the-year. Combined with PAS tactics, you would be fighting a losing battle.

Soo many similarities in your case vs mine. DH got told by a lawyer that it's last resort for court too. 
SS10 is the one that BM doesn't want though and SD is her golden child that she constantly threatens she will take full custody of. Empty air of course. 

It's all classic narcissistic behaviours and we have documented all her PAS with SD and manipulative tactics too. Just in case. Hopefully this social worker does see BMs house and see her home is the one at fault and not yours. Keep up your routine and document everything with Times, dates, ect. 

BM has told SS she thinks homework is too much pressure for the skids to do and doesn't believe in it. She of course will tell anyone in the public something completely different as she cares what people think of her and lies about her image and values.

She also tried to diagnose SS with autism to defend her parenting of him. When the school rang DH and said BM has taken him to a doctor and gave them Paperwork with the diagnosis, DH straight up told them it was crap and he threw that paperwork away. He backed it up and told the school he took SS to a well known recognised pead and confirmed that there was nothing wrong with him medically. SS problem is classic bad behaviour and since he's been in our care more it's stopped. BM of course hasn't responded to the school and they hear crickets from her now. 

It's not easy dealing with HCBMs like this so using nanny came and documenting will protect yourself from her false accusations. 

is in court using a judge as your bludgeon.

Get her to court. Find a lawyer that will go for her throat and quit dancing with this idiot lawyer and this toxic BM.

Indecision is the foundation of chaos and your idiot attorney is profiting from inducing/guiding your indecision and chaos. Find a shark lawyer who will work for you and your Skids and bury BM.

File, get in front of a Judge with the Social Worker reports and get them on the stand. Take the attendance and school performance data and use it to disect BM and her crap.

There is no sure thing in court, but if you do not go for BM's throat, nothing will change.  Something needs to change. Use all of the legal, financial, social, and data tools at your disposal to end BM and protect  your SKids and your family.

I would.  DW and I did. Our son, my former SS-30 (he asked me to adopt him), put the shallow and polluted end of his gene pool far behind him and is living a life of character and quality.  We made their idiocy so expensive and painful that they have for the most part stayed under their slime covered rock at the muddy bottom of their shallow end of his gene pool.  We have always kept him abreast of the CO, the facts, the arrest records, etc.... in an age appropriate manner since he was in his late single digit ages through his teens. He started doing his own research into the lies and crap from his SpermClan that did not pass his smell test.

GIve your Skids the facts. They need them to protect themselves from BM as they progress through their teens and into and through adulthood. BM will never stop her crap. The Skids need 

Good luck.