Lowering expectations of how supportive your relatives will be toward you should be accompanied by lowering the effort you're going to put in to being there for them. Otherwise you end up feeling cheated.

If you're going to join your sister and her family for some social occasion, your obligation is to be courteous while in their midst. Courteous doesn't require pouring your heart and soul into bonding with anyone. It does mean not snubbing anyone by ostentatiously ignoring them. So you don't want to stiffly refuse to exchange a greeting with your nephew. If he says "Hi," you smile and say "Hi" back. You keep your demeanor pleasant, but you don't drop your guard. If he tries to talk to you, you listen with nothing more than a mild degree of interest, but you don't turn away. You certainly don't have to go out of your way to engage with him. Assume he has no interest in what you might have to talk about. Imagine he's like a stranger sitting next to you on an airplane - a stranger whom you find somewhat off putting.. You politely acknowlege his existance, but you stay within your own space, and you don't get drawn into his.

There's no need to let him know you've been hurt by him. That would be pointless, if he's demonstrated that he doesn't care. It might even give him a sense of being one up on you. Far better and more liberating to take the attitude that what he thinks of you is of utterly no significance to you one way or the other. Good luck.