commented on Savage Love Letter of the Day: Straight Uncle Offers Trans Nephew A Summer in the City (But Mom Says No)
If you are the parent of a child who is at risk, for any of a large variety of reasons, it is not unreasonable to be cautious about who you allow/encourage them to spend time with, and where. I say this as the parent of an at-risk child. Of course, you have to balance protection with opportunities to make mistakes and learn. Even with a "typical" child, this is not easy to do, so don't trash a parent for struggling with it.
I am not the parent of a teen - yet - but I have taught thousands of them. I have known 16-year-olds I would trust, say, to take care of my home while I was away for a week, and I have known 16-year-olds I would scarcely trust to get me a soda from the machine down the hall. The balance I mention above is particularly difficult with teens.
I think inviting the family for a weekend or a week during the summer would be the best move. That way everyone is involved in the same experience, Mom is reassured that her brother is not trying to pull a fast one on her, nephew is exposed to some new pathways, and cousins get to spend precious time together - important even if there is an age gap.