I am jealous of my sister being married and getting sex. Sometimes I dream of being naked for her husband who is very dishy, and lately I am thinking of wanting his baby in secret. If he asked me for a fuck, my panties would be off immediately and I would give myself to him without hesitation. I know it is wrong, but how do I stop wishing it to happen before I damage their marriage. Twice I have walked out of my bedroom with just a tiny bra and thong on to go to the bathroom, and his eyes checked out every inch of me.

Don't even think about it.
You could ruin your life and theirs.
It is just a silly fantasy.

I also do not recommend that you fuck your sister's husband. There's no upside, and it could destroy not only a marriage but also a family.

You could flirt a bit if you cab be subtle and leave yourself "an out" so to speak.

I never really flirt with my sister-in-law to any degree. Once I asked her for some antacids before bedtime and she handed them to me through her bedroom door totally naked. She totally did it on purpose- no question.

I'm convinced lots of women enjoy revealing their bodies to "safe men" under the right circumstances. In that case, what was I going to say to anyone? I peeked at her? She was naked in her own room and I stared? Of course not. She did it in a way that was safe for both.

Just a bit of exhibitionism with no liability.