Would you ask about this?

I would ask your sister if there is an issue. if she wont tell you then there's not much you can do about it i guess. maybe if he doesn't want to be there (for whatever reason) it's better that he's not?

I know it might be hard and though the curiosity would almost do me in, I wouldn't ask directly - I think that's a bit too confrontational for my style.

I'm more likely to mention to my sister that he seems to have had a great time at recent family do's so if he decides to change his mind that's cool - just let me know by X date so I can make sure he's got a room.

Then I think I would ? after that.

not everyone's way of dealing, I know, but that's me.

I'm with Mel. I wouldn't be asking direct.

Not much point asking if he will change his mind, as have been told categorically that he isn't coming. It's fair enough that he doesn't want to come but I can't help but feel it must be because of something we've done. Maybe I should have tougher skin but it's upsetting me. Can't see my sis giving me the whole story though.

I would ask directly, but phrase it in a "this is bugging me so I wanted to just check" sort of way. If you focus on how it is making you feel then it would be less confrontational. This is just my style tho.