Whoa. So his mom wasn't talking to him, but his dad (your brother?) is still his biggest champion? I have to say....he does have some special form of audacity. I had a toxic relative, but she had some issues; she invented very untrue stories about me and I washed my hands of her. Luckily, those that mattered knew they were untrue. I made the decision to avoid her and not reach out. Sure, I heard about her, but unlike your relative, she stayed away from me (and played the martyr for it, but oh well). She ended up self-destructing and is no longer alive, but she was in denial until the end. She wanted an audience, not true help, and I suspected that she stayed away from me b/c she knew I would call her on her BS.
And actually, I think that's what you should do with any relatives who keep pressuring you to reconcile. Make it VERY uncomfortable for them to bring it up to you. Suggest that you've been hearing that all he's doing is hitting up people for cash and supplies because he can't support/provide for his own child. And if that's the case, the child deserves more than that. Perhaps THEY should think about what's best for the child; if they have to continuously hit up family to provide even the basic necessities for the baby, then THEY should think about relinquishing their parental rights to another loving couple who desperately wants AND CAN PROVIDE FOR a child. When they ask you for help, put it back on THEM, as the parents.
Now, I'm not suggesting that you say this to the guy who's cut off (continue to ignore him, call the cops, whatever), but to any flying monkeys that may try to guilt you or sing his praises. Of course, this means that you'd have to actually engage a conversation on the subject. But just maybe that would be the thing that would get them to stop asking you: make it VERY uncomfortable for them to bring it up further, because they now know that you're talking about a WAY different solution to them not being able to support their child. Rather than you just handing over money, they should do the right thing and give the child a better home than what they can provide. If they say, "Think about the baaaaaabbbbyyyy!" then you can say, "Yes, I am, and I think the child be better off with a family who can support it without having to harass relatives whom they have grievously wronged." You could even go one step farther and offer to call an adoption case worker/social worker for them.
Right now, you're not making it uncomfortable enough for the flying monkeys who are harassing you. Make them uncomfortable with your responses, and I bet they'll stop the harassment/pressure.