My sister has a friend that I think is beautiful. Unfortunately this woman already has a live in boyfriend.?

When I met her she caught me a little offguard and I'm afraid I did not give a good first impression. Still she seemed to be interested in knowing a lot about me. My sister tells me she has been living with her boyfriend for a few years but they sometimes have a rocky relationship. I will see this woman again at my sister's wedding in a couple of months and she will probably bring her boyfriend with her. I told my sister if she becomes available before then to let me know. She said she would but otherwise does not want to be involved. If she still has her boyfriend at the wedding, what would be the best of letting her know that I am very interested in her?