Warning*** possible trigger for some***

So my husband and I recently took in his 17 year old brother and 15 year old sister last night because their dad left bruises all down the girls arm from grabbing her and she said they he kept pulling her hair ass well. Weve always known he has a temper but hes never physically hurt them. After my husband picked them up which their father said they could go to our house for the weekend. His mother flipped shit and basically said she packed their shit and dont want the kids back and that me and my husband have 2 kids now. * which we are ok with weve been ttc forn 2.5 years and we want them safe* she said she was going to file that she doesn't want custody of them anymore. And I just was looking for tips, help, I have no clue how any of this works with guardianship or adopting ? Idk what's what? Someone give me tips please.