Oh my goodness Fred, what a horrible and upsetting situation for you. It is awful anyway but the fact that you and your bro used to be so close makes it sadder still.
I think, as the others have said, that you just have to leave it for now. You have done everything you can to make everything good, offering all those options at your wedding to make it easier for her, being completely understanding in every way (and yes I also thought how did she cope with her own wedding????). Obviously it is awful whatever she went through as a child but clearly you have been nothing but supportive.
As for begrudging you attention from your family after multiple miscarriages, well anyone who says something like that is surely a little unstable. If she is having a similar experience herself then I would have thought it might help for her to talk to you, or if she doesn't want to talk about it or for anyone to know then she shouldn't be making an issue of what you went through at all. And if she is having problems she really ought to be heartened that after all your rotten luck you did finally get your lovely DD (although I guess it can be hard to think like that if you are not getting the same results).
Anyway, I feel sorry for your bro as he is in an awful situation, obviously having to decide between his wife and you, but you have done nothing whatsoever wrong, honestly nothing, and for him to tell you that if you tried to call your SIL she would hang up on you is appalling.
I am . I know I am talking crap here going on and on but I do think you need to leave it and, if worst comes to worst, just enjoy your lovely wedding with the rest of your family (does your mum know all of this?).
I am rambling .