What's a good excuse to not attend my soon to be sister in law's bachlorette party?

Explosive diarrhea.  No one can argue with explosive diarrhea.  I'm ashamed to admit I have used this excuse in the past and probably will in the future.  Like for the birthday party I'm supposed to go to for my father in law next Friday.  I am seriously considering to claim I have explosive diarrhea but my husband knows I use that excuse when desperate.  If you don't want to say "explosive diarrhea"  try using "stomach problems" while looking embarrassed.

Built-in excuse..."I really don't feel comfortable being around large gatherings yet."  Unless, of course, she knows you've been to other large gatherings, than that's out the window.

Actually, the best response is, "I just can't make it."  All you really owe her is an "accept" or "decline."  If you elaborate, you are just giving her an excuse to take issue with your reason, and to have it thrown in your face at a later date.

Not that I'm assuming she would be that petty or harbor a grudge for a perceived slight.  But that fact that you seem to be desperate for an excuse tells me that you are concerned that somebody will be bent out of shape if you don't go and don't supply an "acceptable" reason for not going.  Maybe not her...maybe your brother (whom I presume is the hubby-to-be) or other members of either her family or your family, depending on where you and your brother fall on Mom and Dad's pecking order.  And don't kid yourself, and parent who says they treat all the kids equally are full of ****.  There is a definite "food chain" among siblings.

If it were me, I would just go with "I can't make it."  If future SIL had the chutzpah to press me for an excuse, I would make up one so outrageous that she would know she's being told in so many words, "None of your business." 

For example, if you live near Pittsburgh, like I do, it could be something like "I'm sorry, I've got tickets to the Pirates game that night."  Because let's face it, any bride-to-be who is told somebody would prefer going to watch those sad-sacks to attending a bachelorette party would have to know that they are not going to get a straight answer from you.

i don't like you

Gathering with covid19 around isnt wise that's been a trending excuse for a year...you could get exposed at work.

You have a migraine and started your period that a last minute excuse.

Came home from work sick...diarrhea 

Humm, ...lots of options...

I think instead of the excuse just return invite with a decline...you have something on the calendar. 

Tell her you have a date, then go to dinner with her fiancee.

The proper way to respond to an invitation is with either "Pleased to accept" or "Regret I'm unable to attend." It idea is to give the hosts a headcount, not to share personal information. It's impolite to pry into why people decline ones invitation, the form of rudeness known as Begging To Be Insulted. If someone tries to quiz you on why you can't accept the invitation, you give answers that say nothing like "It just doesn't work for me" or "It's not possible" or "I'll let you know if things change." If you give reasons, then they just argue with your reasons. 

Tell her that you are having to quarantine / shield with the 'Virus' about. 

Well obviously you aren't that close to your sister in law if you're willing to lie to her. Better just tell her the truth and go on your merry way. 

Tell her you have a date, then go to dinner with her fiancee.

How about: 

'There is a covid pandemic going on, so it is not safe to go to any party this year.'