VERY big problem please help?

my sister died 6 months ago and left behind her 7 month old daughter and her abusive and alcoholic husband. my husband is 24 and im 22 and we have a 2 year old and im 6 months pregnant with twins, my husband and i would be more than happy to take her in but my mom said that ryan will fight back and it will scar elly. i on the other hand do not think so. i am willing to fight for her so i have these questions: i DO not want to replace my sis so should westick with aunt adrianna and uncle marcus or should she call us mom and dad? should i go to court for her? how should i manage a 2 year old and twins plus an additional baby? what could go wrong?