What can I do?

Hi ladies.

I'm 22 years old, and have a twin sister. She had a baby when she was 16, and during that time, she was being sexually assaulted by a person who was supposed to be looking after us. The baby is not his, however she does not know who the father is. Her son is 4 now, and due to the circumstances around his birth, my sister chose to give him to our mum, she now has full custody of him, and my sister sees him occasionally. I recently had a baby, who's now 6 months old. My baby, and my sisters boy have a very close relationship, and Mr 4 year old noticed that my baby has a dad, and started asking questions. He said to his mum, "mummy, do I have a dad?" in front of us all, and my sister didn't know what to tell him. I told her that she has to find out who the father is, there's 3 people who she thinks may be. My sister- psychologically, the sexual assault fucked with her, she's more immature than I think a 22 year old is, and finding out who her boys dad is, is not on her list of things to do. She's been avoiding it since the day he was born. I want to know if there's a way that I can get DNA samples from these guys and do it myself. It's so important for her boy, not to mention a guy who doesn't know he has a son out there..