Not at all. There’s one simple rule here: you have permission to do anything that’s safe and legal, to stay sober. Saying you’re sick and can’t go, is totally safe and legal, so it’s ok.
Another thing is, in a way, you are sick - we all are, in this way: we threw our lives completely off balance and poisoned ourselves with our addictions. For that reason, we can absolutely say that we can’t go because we are sick. (More importantly though: you are not going because the event is alcohol-based, which is literally a bunch of people pouring poison down their throats. But that’s maybe not a perspective to share with your sister - that’s just for you to understand you’re doing the right thing, even if she doesn’t understand.)
“I don’t want to sound like a prude” - I totally get it. At the same time, one of the things we need to do in sobriety is stand on our own two feet, focus on what we can actually control (and not what we can’t control), and not let our lives be defined by (our assumptions about) what people might think about us.
You can’t control other peoples opinions or perceptions, and you never will be able to. It’s time to let that go. Of course you may worry about things, or feel unsure, and that’s normal - but feel it, then let it pass, and don’t let that thought push you in directions you know, instinctively, are dangerous. Trust your gut - it’s telling you not to go, so don’t go, and have faith that things will work out. (And while you’re not going, maybe you go to the gym and get a good workout in, or go for a walk. You’ll feel good about it 
)
“She is a very judgmental person” - yep. That happens. Again, it’s not something you can control. The power of her judgment only comes from you letting it control you. As you work on your self-awareness and self-care (in your sobriety, in getting grounded and functional, in discovering yourself and not drowning yourself in booze) - as you get more self-aware and confident, you will find her judgment has less and less influence on how you feel, until finally, you are free: you will be “riding the wave” of your life on your own, without being thrown around by the challenges life hands you.
You’ve got this. Remember, one simple rule: you have permission to do anything that is safe and legal, to stay sober.
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Very hard to explain but she would tell me to “suck it up” I have taken a Covid test which was negative along with the kiddos! Hard when you have 3 kids in 3 different schools bringing me their germs 
doesn’t matter if you have any excuse or not, if you don’t want to go don’t go. listen to your gut!
