I'm going to feed my vegan nephew meat
Would your sister be okay with that?
I would tell her that he's struggling with eating meat, yet he should overcome his weakness.
Unless your nephew is being forced to eat a vegan diet, this is not your place. You certainly wouldn't like someone tricking you into going against your values.
It's okay, I can convince her I'm right. I'm going to make him a steak soon.
I really advise against this. Your nephew's way of living shouldn't be your concern.