Oy vey. I'm sure you're going to get the full Scarlett letter treatment around here, so let me be the voice of kindness. I'm sorry you're going through this. It sounds like social and religious values put you into a difficult circumstance, combined with getting married very young. I would recommend Dan Savage's podcast which covers a lot of gender and relationship topics.

You should be very proud of coming out, whether here or anywhere else. That takes courage.

Yes, you should get divorced, and allow your husband to find someone who is capable of making his dreams comes true. He deserves to be loved and cherished. If you're going to pursue a relationship with his sister, who sounds like the love of your life, then I would wait at least two years before telling him or anyone else. I can't help but worry how hurtful this will be to him, so put him first for a long long time. You have both been thinking of yourselves, it's time to think of him.