I Have Social Anxiety

So soon it's my brother in laws, dad's birthday and they are hosting a huge birthday party at a place they have booked. My sister is trying to get me to go, but she doesn't know I don't do well in crowds and it'll only frustrate. I'll just be a buzz kill, but how do you tell someone that's why you can't go, it makes me look so weak and I know they'd all have some crap to say about it.