I've just re-read my rather long message below - I'm sorry if I sound like the bringer of doom, but I hope my words give you a useful perspective. Good luck - MrsTHxx
--
Wow, Blueskythinker. What a complicated situation. Why do you think she's been so awful to you in the past? Has there been a long-term problem between you which has never been resolved (i.e. something that started before you had a baby 3 yrs ago)? It sounds as though there may possibly be something which she's aware of, even if you're not. It's very rare that such animosity springs out of nowhere, and it's even possible that you may have done something innocently which may have hurt her a great deal.
As for your BIL, he sounds like a total moron. Is he totally incapable of independent thought? His responsibility (were he mature enough) is to play a conciliatory role - supporting his wife within reason, of course, but ultimately aiming to bring you both together for a thoughtful heart-to-heart. He deserves an almighty slap for inflaming the situation. I only hope his LO's don't learn from him.
Think about your life in 30 years' time. Children grown up and separated from their cousins because of silly bickering between their folks. Not only that, but your children will end up falling out with people they love and have no understanding of how to handle it maturely - because YOU chose not to teach them how to do it when you had a chance. Not only that, but imagine that you and your very own flesh-and-blood sister are STILL not on speaking terms 30 years on, and probably can't quite recall why. Is that really what you want for your lives?
At the moment you have the luxury of imagining your lives 30 years on. But what if you don't live that long? I have lost 3 members of my immediate family (we used to be 6) - two of them died aged 56 and 35. I can tell you now, hon, that when people die, that is it. No second chances. And no matter how pure the hatred you feel towards your sister today, if she passes away first, you will grieve and grieve and you will never be able to mend that relationship. I tell you this from personal experience.
I also had a very difficult birth 17 months ago, which left me in lots of pain for the first nine months. In a couple of weeks I'll be having an op which I hope will finally sort me out for good. My body has been such a source of stress and trauma for me as a result of that birth, and if I'd had a sister (only brothers, no mum around anymore either) who had come around and given me any kind of support it would have meant the world to me.
I hope you can see now that this is not just a straightforward falling out between two people, but an intense and complex situation which can potentially have a lasting impact - for good or bad - on you, your sister and even your own children. I know things have been AWFUL and in one sense why on earth should you be the one to make the first move? But on the other hand - what an opportunity. Are you really going to let it slip by?