You have every right to feel hurt. First off you lost your sister and are still grieving. I'm sure you expected your BIL to be doing to same, and to concentrate all of his free time on the sons left to him by your sister.
Sometimes there is no explaining people's actions. Maybe he felt he needed help to raise the boys and couldn't do it himself. I know it seems very selfish to stay away from them when they needed him the most.
But, I think since he will do what he will do anyway, you should stay in good standing with him so your family can be a strong unit for the boys as they grow. I would hate for you to lose that.
Good luck
