First off, I'm glad that your sister accepts you!
I don't have much experience in coming out to people ( I've only told one friend in real life and just a few on the internet ) but hopefully I can give some advice.
Whether or not to tell your sisters husband...
There are some general questions and tips you should probably consider:
-First off, what are his general political stances, or if he's ever mentioned a current event relating to anything LGBT, his opinion on the matter may be able to show you how he could potentially react.
-Second, what do you hope to gain from telling him, and what do you have to lose? ( this one you've probably already answered, but it can't hurt to ask yourself again )
-Third is timing, especially since you mentioned that he and your sister are having relationship issues, maybe it's not the best time to bring it up to him. It may add complications to the relationship between him and your sister.
-Fourth would be that I recommend talking to your sister. She most likely has a general idea already of how he will react and will probably have the most solid piece of advice on whether or not to tell him.
-And last is that he is not entitled to a news flash about your gender identity just because he is married to your sister. Telling is completely up to you (though I would still discuss the issue with your sister). You can tell him when you feel
safe doing so and when you know for sure you are ready for him to know.
It's not much to go on, but I hope this advice helped. Best of luck in whichever decision you make
