You did exactly the right thing!!! Your job as a part of your nephew's family is to help him become the best person he can be and he can't do that in that environment! If he's mad at you SO WHAT? He'll appreciate it later or he won't. That's not your problem.
But if no one tells the Mom anything because they're afraid someone will be mad or not like them then we have no chance to help our kids become someone they can be proud of! I tell my kids all the time "I'm not here to be your buddy, I'm here to make the tough decisions that you don't have the experience and knowledge to make alone. I love you enough to still do the right thing even if it means you are mad at me. I can live with you being mad at me, I can't live knowing I was afraid to do what I knew was right!"
Hang in there, don't judge your nephew but continue to let him know you will always be there to try and protect him BECAUSE you love him!
But if no one tells the Mom anything because they're afraid someone will be mad or not like them then we have no chance to help our kids become someone they can be proud of! I tell my kids all the time "I'm not here to be your buddy, I'm here to make the tough decisions that you don't have the experience and knowledge to make alone. I love you enough to still do the right thing even if it means you are mad at me. I can live with you being mad at me, I can't live knowing I was afraid to do what I knew was right!"
Hang in there, don't judge your nephew but continue to let him know you will always be there to try and protect him BECAUSE you love him!