I think you should write your sister a letter. This way you can be careful what you say, and you won’t be putting her on the spot.
It would be a real pity if there was just a misunderstanding that has caused this. Or perhaps she has a real reason, that is easy to fix.
If she doesn’t want to offer any reason, she can just ignore the letter. And if you think it’s better, you could state in the letter that while you would love an explanation, it’s not necessary for her to respond if she doesn’t want to. Or she can respond whenever she feels up to it.
If you word it right, you won’t offend her. And no harm will be done.
I don’t agree with those who say leave it alone, just be happy she’s happy. You can be happy she’s happy, but if something can be done, why not try? Futhermore, she may not be happy, but doesn’t want you to know it and that’s why she is keeping away. (Obviously you should take this possiblity with a large grain of salt, since of course I don’t know that is true, just throwing out a possiblity, since I know people who made believe they were the happiest married couple, while nothing was further from the truth.)