Bridesmaid's Betrayal

Yikes! I would ask her to step-down or change her attitude. If she doesn't then I'd tell her she's not int he wedding party any longer.

I don't see how this qualifies as a betrayal. Trying to seduce your fiance would be a betrayal, not skipping out on a party. While she didn't handle things very well, bachelorette parties are completely optional and not attending isn't a reason to kick her out of the wedding.

First of all, I'm sorry you're having to deal with it. It sounds like there's much more going on with her than meets the eye and it could be a number of things, from envy or jealous issues that you're getting married to maybe she's super stressed at work. Either way, as a bride, i wouldn't deal with it...because it will stress you out continually and the difficult challenge is if you continue to make concessions for her and asking others in your bridal party to do so too may build resentment and you don't need that kind of negative energy around you. You can "relieve" her of any "risks" and let her know that due to her concerns, she is still welcomed to attend as a guest since she feels like one of the bridesmaids is "contagious". It won't be an easy conversation and if you're direct, you may just have to ask what TF is her problem... It seems like it could be something going on in her world that has nothing to do with you but her lashing out is still no excuse.

Yeah, it's a weird time right now - I know some people who think the whole thing is a hoax, and then others that seem like they are preparing for some kind of apocalypse. She may be kind of flipping out about that, or potentially about financial issues as many of us have taken a major hit in the markets with all this as well.
It's rough, it doesn't excuse her being rude, but it may not be worthwhile to damage a friendship.

I'd find out why she behaved the way she did and ask her to kindly apologize for her behavioral. If she's unable / unwilling to do so, I'd ask her to step down. What does her sister say?

I work at a college (so like a place where people are VERY concerned about coronavirus) and have several friends who work in healthcare. The hysteria around Covid19 is very real right now, and people working in healthcare sectors are dealing with a lot. How far off is your wedding?

If it's pretty close, I think a conversation with her "hey, how are you doing? you seem stressed? I care about you" would be well warranted. She might just be under an incredible amount of pressure right now because everyone is freaking out about a virus that for most is just going to be like the flu hopped up on pre-workout.

If your wedding is still some time off I would just give her and the whole virus hysteria some time to settle. It might blow over within a month or two, or the situation may become more serious. Only time will tell.

I agree with PPs that we're in a crazy time right now, but stress about world events/etc. doesn't give you an excuse to blow off a friend. We still have to be good people, even when we're scared or stressed at work. If she wasn't comfortable coming, that's totally fine and understandable, but ignoring you and refusing to talk about it is childish. I think having compassion for her fears and concerns, but taking a strict approach on how she treats you is the best option. I'd give her sister a heads up, but then personally I'd send a message showing compassion about whatever it is going on in her life (don't make an assumption that you know it's about the virus or anything else), but then ask/tell her to leave the wedding party. Good luck with this!

I wouldn't kick her out yet but I'd tell her the two of you need to have a talk to get on the same page. I understand the coronavirus is making people panicky and uneasy so that may have been her issue and she wasn't able to properly express it so that you understand. If she doesn't change her attitude when you guys talk and is still blaming others for her actions, I'd ask her if she's still interested in being in your wedding as you need people you can count on on the big day. You'd still love to have her as a guest but maybe her being in your wedding party isn't the best fit.