Should my brother be allowed to go through with this?

So me and my Aunt who my brother lives with are struggling with this. He's 17 turning 18 in October. He and his long term girlfriend, she's 18 turning 19 in June, broke up around December. Two weeks after that she told him that she was pregnant but around the end of January, she came clean and told him that it wasn't his but belongs to someone she cheated on him with. She's been showing up at their house every day begging him to talk to her and she told him that he lied to her because the actual father is a terrible person who deals drugs (Its true actually, we all know him) and she knew my brother would step up & love the baby. My brother told us that since the biological father wants nothing to do with them, he wants to be there for the baby. My Aunt told him hell no and he told her he'll be 18 when the baby is born so he can't be stopped. He's currently trying to pick up more hours at his job to make more money so he can support his ex and her baby. We keep trying to tell him that he shouldn't be wasting his time with this situation at 17/18 but he told us that if we keep trying to force him to do what WE think is right for him then he will push us away.