Step parent help

My 2 oldest children were places into abusive situations due to bio mom 2 men who abused my kids who she left twice and than went back . First man she married left twice my son suffered physical abuse my daughter mental ..

Second man- left twice physically snd mentally abused both my kids,

In all 4 situations I helped bio mom leave i gave her money I let her lean on me trying to show good co parenting and a good example for my kids

Last round when bio mom chose to go back my kids said no they wouldn't return to the abusers home bio mom kicked them out

After 2 years of toxic relationship between kids and bio mom, lots of manipulation ect she gave up custody not even a month after signing paperwork she is now trying to convince my son to move back in qith her part time.... he is so desperate for his bio mom he really wants to do it, I dont want to say no but I have no trust in her, she wont communicate with us, shes just toxic putting my feelings aside I want my kids and bio mom to still have a relationship but I need to do right by my kids.... what do I do.....?