I love just knowing he's close by me. I just don't want my romantic inclinations toward him ruin my friendship with her or my friend. Also, I'm not sure how his sister, my best friend, will react. It's agonizing to think I can't be with him. At least, you're assured of a wedding invitation, even if you won't be the one kissing the groom and exchanging lifelong vows.
Good friends are too important to carelessly push aside.
Start by talking things over with your good friend. If anything, she's the one who could most easily let you know of any potential. Of course, this is assuming she's cool with your feelings for her brother. As for doing anything else, until bringing it up to your best friend, making any aggressive moves is too risky. There's no reason to agonize.