Your nephew needs you. It takes a village to raise a kid.
This is your sister's failure really, not yours, this is her "well, crud!!!"
But it probably does make you think what your life would be if you did have a sister who was kind, responsible, caring, sober (not needy). That is honestly what I thought about when my husband kept relapsing. He has been sober for over 5 years now. I just see him for now for who he really is. It is not perfect, but my kids have their dad completely.