My fiance has been struggling a lot with his relationship with his sister and that has started to put strain on his relationship with his family.
Here's the background, he and his sister used to be best friends and used to talk about living nearby and raising their kids together. She has gotten married and moved a bit further away ~2hrs and really since then the problems with them have started. For one she hates coming to visit and makes that really clear whenever she does. Apparently her husband isn't a big fan of "family" and I'm not sure what that means to him but that is what she says as an excuse every time she cancels. Oh and she does cancel frequently. Even when the plan is for us to drive to see them things get canceled. For example we were going to go there to have a pre-thanksgiving dinner since she was going to her husband's family for thanksgiving and she canceled that the night before. We ended up not seeing them until February even though we had made multiple efforts to find time in between. His strain with his parents is because they don't seem to feel this is wrong in any way while he is deeply hurt by being left out of his sister's life.
It's gotten to the point where I see him so excited to see his sister and I know that there's a really good chance she is going to cancel the night before and he is going to be devastated. Should I try to talk to his sister and see if maybe she doesn't realize how much this is hurting him? Or is this really none of my business and I should just stay out of it and support my fiance when he is hurt?