Wow! so much gone on while I've been away. Most importantly the "show". Keep us all posted . Like Pruner's response too.
Try not to dwell on the fact that you think your sister hates you. Hate is, after all, very akin to love in a way. What I mean by that is that no-one would waste their energy hating someone they couldn't care less about would they? Emotions can be so complicated.
Perhaps she is jealous, with or without good reason. (You don't actually know what went on when you arrived as a baby and kids sometimes have a very distorted view of things which only gets worse as time goes on).
I really do hate judging anyone, honestly, but it's her DH that I'm really beginning to think is a bit dodgy. Sounds like a bit of a misogynist (sp?) to me as he's not very interested in his daughters and is so set on a son. We can't see what goes on inside other people's marriages, even with siblings, and how are you to know that he is not insisting on "Thomas" and that her belligerence is not just a defence mechanism? She could be feeling that between you and hubby she's never going to win so she might as well go along with the one who has most power to make her life easy or miserable?
Also, did your mum's worry about how you would react come after a suitable amount of care, concern, pride and pleasure at your sister's achievement? It would be a bit awful if your sister said "I've just had boy and we're going to call him Thomas" and your mum's first reaction was "what will your sister feel about that?" Not trying to put the blame on your mum btw, just wondering. See, there's so much we don't actually know here.
I know she's rich and got nannies etc but she is far away from her family with (it sounds like) a not very caring hubby and she must be a bit jealous that your boys see much more of (and therefore are probably closer to) their grandparents than her girls.
Hence her having a go at you for using your mum so much as a babysitter?
You say they're not being bothered by loads of visitors because they have no family over there? I think that's a bit sad for her. Or am I strange in having enjoyed all the visits and attention I and my babies got when they were born?
All that notwithstanding, I still do understand how upset you must feel about the whole thing and you do have my sympathy, really.
Try to concentrate on your imminent confinement. I'm so excited for you. Lucky thing! There's nothing like the birth of a baby is there? I wish I wasn't so b*dy ancient!!! . Ah well, just have to wait for grandchildren!