I am hoping I understood this correctly:
1) your sister has four kids with four different men
2) three of them maintain contact with their kids, but one does not
3) you are "good friends" with the one who does not maintain contact
4) your sister has a new boyfriend who wants to play daddy to the neglected child
5) #4 bothers you
If I have this right, there is a helluva lot more to be worried about than you think. YOur sister has some major issues, but they are for HER to worry about. As for you, why on earth would you want to be "good friends" with someone who would do that to his own child?
Relax... I'll need some information first. Just the basic facts - can you show me where it hurts?