Mom and sister always fighting over who's the father?

This whole thing is pretty complicated but here goes. My older sister Jessika (18) is currently at war with our mother. Mom wants to know who the father of her 3 kids are. My nephews are 5 and 2 and my niece is 4. I know, Jessika effed up really bad back then, but let's focus on now. Jessika was 12 when she first got pregnant and she wouldn't tell anybody who got her pregnant. My mom tried every tactic to get her to talk, but she wouldn't tell. I umed that either A) she didn't know who or B) she was being abused and couldn't tell us. My mom eventually just dropped it because she was stressing Jess out. So 3 months after the first one, she gets pregnant again and moved to her dad's house. Once again, no father around and wouldn't tell us anything. My mom monitored her every e-mail, text message and couldn't figure out how this happened again. It was so weird because none of us had ever even seen her with a guy. She was 15 when she got pregnant again and mom kicked her out for good because she wasn't "learning anything." Jess lived with her dad up until she moved into her own apartment last year. I don't get why my mom is still so hung up on who the dad is. Jess is the one that's going to have to explain the situation to the kids not mom, so why put so much energy into it? Jess probably will never tell her and my mom is starting to wreck the already damaged relationship they have by always bringing this up. I mean, Jess takes care of the kids by herself for the most part and is a good mom, so why do this?