She could have a crush, or alternatively, she may have some sort of, not-unheard-of, sibling rivalry... Or in another twist it could be that she really looks up to her sister and thus desires the approval of her sister's mate. At any rate, something unfortunate is taking place.. Unfortunate for you, anyway, as no matter how you broach the topic you're bound to come off insanely jealous and like you've lost the plot. I'm probably more manipulative than you but I'd try displaying similar behavior towards her sister and see if she notices <- Disclaimer: bad advice, but if you do try, tell me how it goes!!!
“Love is the ultimate outlaw. It just won't adhere to any rules. The most any of us can do is to sign on as its accomplice. Instead of vowing to honor and obey, maybe we should swear to aid and abet. That would mean that security is out of the question. The words "make" and "stay" become inappropriate. My love for you has no strings attached. I love you for free.”
― Tom Robbins, Still Life with Woodpecker