How can I make my soon to be sister-in-law comfortable around me?

My boyfriend's sister is a little heavy and not particularly attractive. She loves her brother, and over the past 4 years we have been together, has always been polite and cordial to me. I think that she finds me physically threatening, as I am about 5 years younger, thin and more so attractive. I base this on the fact that the only female friends she is willing to hang out with are her age and also generally heavy and generally unattractive. I would like to have a better relationship with her, and receive more invitations to things with them. However, her seeming distaste for me is frustrating and disappointing. Please help.