Family wants to have a party

What can I do? I don’t agree with it at all. They want to have a birthday part for my sister who is turning 21. There will be probably be 20+ people there. It will be outside but it’s impossible to social distance while 20 people are swimming in a pool. I just don’t want them to have it 😞 I have really bad asthma so I’m trying to be really careful. If I don’t go outside with them all, they’ll all yell at me and spend the whole day pressuring me to join. They never respect my boundaries. Not that it matters anyway since I live in the same house as some. So if they get it, I will get it. But also what if we have it and are asymptomatic for now. My aunt works in a nursing home. I swear people have no consideration for others. It makes me so upset.