I think you should talk with your sister, not your best friend, right away. You didn't say anything about her having feelings for your best friend, so maybe if you tell her how you feel about your best friend she will easily step aside or whatever. I also want to point out that the 17-22 age gap is pretty big for that age- unless your sister is very mature for her age, I would go with your prediction that your bf likes your sister for the sole reason of her being open with her sexuality. She could also like your sister just because she feels like she gets support or reassurance for her recent 'coming out'. If your talk with your sister goes smoothly, I would wait a little while to talk to your best friend about your feelings because if she does truly like your sister, it could take her a little while to get over her. Once that happens- you can move in!