What right do i have?

im 18 in my sister is 26 and has 3 kids.her and her boy friend are on roxy (roxicondon) and the kids dad is on drugs too and his mom is the some way because she give the money to him for them and she give her meds to him and the will the kids are to me and my fiance but he get in a car wrack and did not mike it so what can i do to get them because our mom died 10 years ago then our dad died in june this year then my fiance oct of this year thanks for your help