I got a long very well with the rest of his sibligs--but there is just this ONE sister of his that hasn't seemed to take a liking to me. We often don't even greet each other or say good bye--after years of dating and a recent engagement with my FH--its becoming awkwarder and awkwarder to try and break the ice which probably should have happened years ago.
Funnily enough--at dinners we often end up sitting beside or across from each other. And when we get some alcohol in us we usually share a few funny anectodes with one another.
Now that I think about it...we aren't even facebook friends...
I say just start spending more time with her if you can. Slowly try to build a friendship. That's what I am going to start doing with this sister. And even if you don't end up forming a friendship--all that matters is that you guys are kind with each other.