I've got 3 entire males with no problems at all between them.
My bitch sometimes humps my youngest male lhasa but only if he gets out of line with her

The humping is not a sexual thing, it's a dominance thing and neutering won't cure it. I'm guessing he's been the one in charge at your sisters and this might be the reason she's letting him go
What you need to do if you're going to have him is show him that you are the boss and that the kind of behaviour he's exhibiting won't be tolerated.

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ShazaLhasa
My fabulous furry friends