If this is for real, you need to start living in the real world. The "love" was obviously one-sided from the start. It doesn't matter if he recognized how you felt or not and it doesn't matter if he felt the same way. Your life is what you created, so you either appreciate your own marriage and children and respect your sister's family... or you continue on as you are now, starving your own marriage and effectively ending it.
It's great that you say you don't want to disrupt your sister's life, but it still feels as if you have no issues disrupting your brother-in-law's life. Why? Because you don't ever refer to him as your brother-in-law. He's always YOUR friend or your husband's best friend.
Start respecting the fact that your sister's husband never, ever belonged to you. Respect the fact that while you feel it's you who has "trust issues" and all of that other stuff, it's really YOUR HUSBAND who is the one who ought to have the trust issues towards you. I'd be curious to know what he thinks about all of this... if he has any idea that you snowed him and dated him, got engaged and married to him, introduced his best friend to your sister and encouraged them to marry... all so you could be close to this other man. If the truth had come out in the very beginning it would have hurt no one.
If you want someone to tell you it's all right to leave your husband, that's your choice. You either love your husband or you don't, and you either think it's worth staying with him for your own reasons or you don't. You either learn to love him as a husband, as your partner, as your lover, as your best friend, or you don't. Whatever you think is missing from your own marriage? It takes effort.
You want a friend. Who is a man. Who is married to your sister. That specific man. Well... try your husband. All of the things you want to share with your sister's husband, try sharing with your own husband. Try building a bond with your own husband. He must have some sort of value to you other than having been a stepping stone to the other guy. Obviously he's not any more. So act like you're married to him or let him go.
You also need to start acting like a loving sister because your sister? She's not a stepping stone for you to get her husband either. She never was.
Marriage counseling. And individual therapy for you.
Lastly, if this is a plot line to a book or short story, it's a really bad one. It's too convoluted and the main character is not very empathetic. In fact, I'm angry with her. Try it from the point of view of the sister.