Other than purely an issue of dexerity, the child may also experience some awkwardness if Dad and Sis play one way, but he plays the other. All the 4 years old I have met are a curious mix of trying to fit in and wanting to establish themselves... the chips may fall either way.

Also, if he starts RH and has difficulty keeping up with his sister it may squash his interest. I gave up piano at a young age because I was so frustrated that I perceived myself to be struggling while my brother picked it up like a natural.

I think I'd suggest starting him RH, but be aware that it just may not work for him and let him know that there is another way if he wants to try it.