You're missing the point that he was told he'd get 50% and now the house has gone.
He just wants to ask why?
I see no problem in that to be honest.
At the moment, he's not even 100% sure that his sister didn't buy the house from his mum - if she did, he'll still get a chunk of the money when she passes - he doesn't know it (or it's value) is 'gone' from the equation.
There's so many different potential scenarios that could have played out here, which is why the first thing he needs to settle on is deciding what he actually wants as an outcome from the conversation. Does he actually just want to know 'why?' through a sense of curiosity or does he feel entitled to more than that? It shapes the conversation in completely different directions.