Isn't it horrible when your nearest and dearest are like this OP? I have an older sister who simply cannot help herself with her catty comments, all brought about by the green eyed monster. Right from when I was old enough to start choosing my own clothes the monster started to come out, and she simply wouldn't rest until she had whatever it was that I had. A new top, she had to have the same one, even the same colour, new shoes, same ones, same colour. It used to drive me nuts at that age, and Mum always said 'imitation is the sincerest form of flattery', but I could never see it that way. Then as we got older and had homes of our own, we'd buy a new sofa, cooker, whatever, and she had to have one too. This ranged from small things like clothes and shoes, right through to life changing things, for example I announced that we were expecting our first baby, and in spite of saying she wasn't having any more, 3 months later, she suddenly announced another pregnancy!! Then, if there was no way that she could afford to copy what we had, it would be the downright nasty comments, encouraged by her equally jealous DH. The last one was when she came to visit our new home. She lives a nice lifestyle herself, not rich, but she doesn't have to work, and lives happily with her DH. She came to visit us and because there was quite a long distance involved, we invited them to stay in the holiday let we have, so that they could have some privacy and enjoy a little holiday, a nice thing to do for your sister some would think. They left a day early, with some ridiculous excuse, and I could just tell it was the 'monster' coming out again. Just to see what she would say, I asked her what she thought of our new place. Her answer 'I don't like it!' If it had been anyone else I might have been shocked, but was half expecting it with her, as there was no way she could replicate what we have. I asked what she didn't like about it, and thought she might say, 'oh, it's a bit too far from town' or something like that, but even though I know her so well, I was a bit taken aback when she said 'I don't like anything about it'. At that point, I said 'Oh well, it was nice to see you, have a safe journey home', and off they went. When I told DH what she'd said, he was horrified, but I just laughed, as over the years I've just become accustomed to the fact that she really can't control her jealousy, and just ignore it. The difference between us is that if she gets something new, I'm ALWAYS happy/pleased for her, but she just isn't made that way.
It seems though that you struggle to ignore it, and just be happy with your life OP, so like others have said, I would either bite back at some point, and say something like 'Can't you just be happy for us occasionally? Why do you always have to be so nasty, you have nice things too?' If you can't do that though, then cutting contact is really the only way to go. Such a shame, but their loss.