My husband and his ex have 2 kids together one lives with us fulltime and the other lives with mom, she has a?

Borfriend n another baby he just got out of prison but becuz she lives through housing he can not live there so almost every night of the week they sleep at his moms 2bedroom house the 9year old sleeps on the couch n crys herself to sleep(9 yr old words) the baby sleep with grams mom n boyfriend in room they have no court agreement for visitations or anything she wants to stay n live with us but mom said no could we just keep her if we wanted to or how does that work since there is no custody agreement