`m not a USA citizen, so there are some things that I ignore.
Just can say to don`t let your 12yo stepdaughter be alone with the baby.
You have received very good advice, but probably it would be a good idea to seek help with a therapist.
This people know a lot about this stuff.
About your stepdaughter receiving the helps she needs, in MPO I wouldn`t get there. It’s her mom`s issue.
You issue is to protect your baby at any cost.