How to uninvite family to wedding?

I agree with others that with your wedding 10 months in the future, I would just put this on hold. You have at least 7 months until invitations should go in the mail. At the very least I think a face-to-face discussion can wait until FH's work schedule lets up. I can't believe there is any way to frame a text message to a relative telling them they are uninvited to your wedding in 10 months because of their passive-aggressive actions, not having a relationship with you, and their mental health status, that won't have a very high-risk of being perceived as "rude."

In the meantime, you don't have to engage with her. If she has as many challenges as you've suggested, I'd guess the "many people" she's talking bad about you to tend to take what she says with a grain of salt. Also, there is an organization, NAMI (National Alliance on Mental Illness) that has a wealth of free resources available specifically to help family members of those with mental illness (NAMI.com). If there are members of his family with illnesses, you might both find their "Family to Family" program invaluable. Good luck to you!

Nothing you're describing is worth the relationship-ending drama of "uninviting" the groom's sister from the wedding.

It's much too soon to bring this up with her if you want to keep the drama to a minimum or else you're going to have it for all those months leading up to the wedding. A lot can change between now and then anyway.

It sounds like you're more opposed to her coming than your FH is, which is concerning being that it's his sister. I would highly advise against sending a text message - for reasons previously stated, you have plenty of time for her to correct her behavior and/or have a sit down conversation with her. Sometimes people with these personality types are looking to get a reaction from you more than anything, I wouldn't give it to her... If she's still acting disrespectful within a month of the wedding, after you have already addressed it in-person, I think that would be the final straw to uninvite.

I'm not going to comment on whether this is or isn't the right decision (in my opinion) since that wasn't your question and we also don't know the entirety of what you're dealing with. I will say that while our situation was different than yours, we had to face the ugly reality of uninviting my husband's mother from our wedding. I have a discussion post where I went into detail about our situation, but it ultimately came to an end by my husband telling his mother:

"This isn't up for discussion and I'm not looking for an argument with you. I ask that you consider how my fiancée and I feel and understand that we no longer feel comfortable with you attending our wedding. If you ever decide you'd like to make amends with us, we are open to that conversation and attempting to mend a relationship with you. Until then, I ask that you keep a respectful distance from the both of us and allow us to enjoy our wedding day".

It was difficult to do, but it had to be done. We didn't want it to come to that with her, but we had a great wedding day without her being there and we know that wouldn't have been the case had we allowed her to come simply because she was "family". If you ever need someone to talk to about this, please send me a message. Best of luck to you and your FH