please respond someone. very serious questions.

I am an 18 year old female who was somewhat in foster care and group homes growing up because my mom was on drugs and my dad just wasnt the all american dad...no need to get into details. I have a little sister who is 2 years old. My mom was on drugs while pregnant with her and she had to give up her parental rights. My cousin (through marriage) took on my little sister when she was 2 days old and she is now 2 1/2 or so. Well my cousin isnt really a good mom to her. Shes always at the bar drinking and my uncle who lives with her takes care of her older biological daughter and the baby. The adoption still isnt final yet and one of the reasons the adoption agency hasnt gone through with the adoption is because my cousin who is adopting the baby was raped by her father 15 years ago and she got pregnant by him and had her daughter. Well 15 years later and she still talks to her father and sees him on a regular basis and the adoption ageny doesnt understand why she still talks to him. Well my cousins daughter was also raped by her uncle. Well recently I found out that my little sister is being molested by my cousins daughter. Shes two years old but shes bright and she told me her privates hurt and that amanda (my cousins daughter) puts markers in there. I didnt say anything about it but then she told me again and i had to tell someone so i told my cousin and i used to get the kids on the weekend and now she wont let me see them... and its like she blames me for it. Well i called my little sisters case worker and left her a message to call me tomorrow and im gonna tlel her everything but im trying to see if i can get some answers sooner. I said im only 18...i want to know if that will affect me from getting custody of her? I know i cannot adopt her until im 21.. atleast thats pa's laws i think. But i would want to be her foster mom. I have a great boyfriend. I have a nice home and money wouldnt be a problem. Im just afraid that now she is going to get taken off of them and ill never see her again. Really as long as she would go to a good family i'd be happy cause i dont want her being molested but i'd miss her very much if i lost her. We have a very close bond. Will they hold it against me because of what my cousin did and not let me foster her because im in the family? cause really im not. I only talked to my cousin but i dont really want anything to do with her anymore. I just want to make my sister happy and give her the life that i never had and the life she deserves. Im just so scared. I know what i did was right but now im just afraid that im going to lose her. if anyone has any answers please tell me.