Ahhh okay, so it's most likely a case of once you've crossed the line into 'enemy territory' you're stuck there for the foreseeable future. In that case, if she's someone who likes the drama of conflict as a matter of course, I would agree with Flux that you should use your time to focus on the rebuild with your boyfriend and not worry too much about her or the extended family issues. The answer is yes, it will end, you and she may never rediscover the friendship you had before but if you and your boyfriend do rebuild and things get back to a good place her feelings toward you will mellow over time I'm sure - most people don't hold on to anger forever, especially if there's nothing fuelling it. If by any chance she and/or the family don't get past their negativity then I would say it's down to your boyfriend to address the situation - they are his family, after all.
The only thing I'd question is how much of it was her involving herself in your issues and how much of it was him involving her in your issues? I have two sisters and they are prone to this kind of thing - one shares a problem with the other in perhaps too much detail and then the other goes to bat for her and ends up being the bad guy because she's judged to be getting involved in things that aren't her concern. Her defence is, if you don't want me to get involved don't tell me anything, if I don't know I won't care, so on that basis I'd have to say as much as it's a bit like closing the stable door after the horse has bolted, your boyfriend could help the situation going forward by not sharing the details of your problems with her and/or his family.