BM might be bluffing about going back to court.... keep to the current CO of only supervised visits,
kids can't decide other wise....

Once a court decided a parent is not fit, I doubt they will go on her word, I would get kids into therapy as well, to have a 3rd party who can honestly say these children are being brain washed by their mother, she's not good for them, keep visits supervised

We're in the UK and had a similar situation a few years ago. DH got custody of his 3 boys after court reports which documented bm's emotional abuse of the kids and showed her to be mentally unstable and unfit to be a full time parent, granting her regular weekend/school holiday contact. We weren't happy about so much unsupervised contact but as it was ordered by the court, we adhered to it to ensure she didn't have any excuse to haul us back to court in the future. That was 10 years ago and bm has never got over it, claiming to anyone who will listen that dh is a monster who stole her boys from her.

After them happily living with us for 6-7 years, she suddenly decided she was going back to court to get custody of the younger 2 ss's changed to her. She managed to get her legal fees paid as she wasn't working but we couldn't get legal aid and couldn't afford a solicitor and had to represent ourselves. We found out that the only time a change in custody would be granted is if she could prove that the kids were at risk from living with us. As the boys were healthy, happy and doing well in school and there had been no involvement with the police or other authorities she could not claim they were at risk so that was a non-starter. She then tried to claim the boys had told her they wanted to live with her, but we knew from the previous court case that she had tried to coach and manipulate them to say that and by their behaviour and the things they told their dad, it was obvious she was doing the same thing again. She also made all sorts of promises to the boys about how wonderful life would be when they went to live with her, which she had also been proven to have done in the past.

At the time, the oldest was 16 and she didn't try to seek custody of him as she knew perfectly well there was no way in hell he'd want to live with her so she was just trying to get the younger 2 back, who were about 9 and 10 at the time. Luckily for us, we had older ss as a witness who saw and heard first-hand the lies she was telling them and as he was 16 and agreed to us detailing his account of what he'd seen in our responses to her solicitor and forms we sent to the court, that was further evidence on top of the documented proof from the earlier court case. After a brief interview with someone from CAFCASS (the court-appointed child welfare officers in the UK), they reported that there was no reason to change custody because the boys were fine where they were and not at risk in any way, but that bm should continue to have regular contact. We also completed form to submit to court to report concerns for children at risk, detailing bm's previous proven emotional abuse, particularly of the oldest boy, and referred to the previous court documents containing evidence of this, so we came up with some pretty damning evidence to stop her ever trying to do anything like that again. She was advised by her solicitor to drop the case and had no choice but to withdraw - very satisfying for us, specially when we'd had to do all the work ourselves without a solicitor.

The ss's bm sounds very similar to yours - tells lies to the kids, has a poor me attitude, is very crude, likes to blame other people, can't accept the fact that the court decided the kids were better off with their dad and loves to seek attention and create drama. I would say document everything, keep a diary of everything, interactions between you/your dh and bm, keep all emails and messages, include things the kids tell you she's said or done when they've had contact, take screenshots of her social media as someone else suggested should you need proof of the kind of person she is.

If the kids have been living happily with their dad and you for a long time and are settled, doing well and not at risk, by UK rules there is no way they would be uprooted and sent back to BM, specially if there's proof she's been abusive towards them in the past. The fact that her own mother has such a low opinion of her parenting should carry some weight, and hopefully if it came to a court case would be happy to give her views as relative of and someone who has a relationship with the kids. In the UK, the court would also take into account the kids' views at their ages. During the first custody case, oldest ss was 9 and they did interview him, clearly stating in their report that he had no wish to live with his mother and was felt far happier with his dad, and I know when I left my abusive ex my daughter was 13 and stated she did not want contact with her dad which the court accepted because they deemed her of sufficient age and maturity to make that decision for herself.

There's probably no harm in consulting a solicitor if you can afford it, or it might be better to wait until you've actually received something in writing to inform you what she's going back to court for and then seek legal advice. If she's going for unsupervised contact after having supervised for a long time, the court will want to know what's changed. She can do all these things to try and convince a court she's a new person but judges aren't stupid and know the tricks people pull. That's where your evidence will come in, plus the views of the kids so document, document, document!

Good luck