SHe shouldn't be around the other kids at all if she's sick. I would warn everyone that shes sick before they bring their kids. Wow.
ANd no, she should not swim. She should not be around other kids right now at all. Sorry, but I would reschedule the party if I were you.
I don't think anyone here meant to be snotty-- in fact I edited my comment before posting it so it would be as gentle as possible. I think a lot of us are in shock that someone would have a sick kiddo at a party at all. I think the anger is because it's happened to all of us-- running in to the family with the sick kid at the store or at playgroup or, at a party. And I think we all would appreciate a heads up when that kind of encounter is going to happen. Sick kids shouldn't go to parties. Or playgroups. Or school. The rest of us would appreciate it. It's not fair to expose other kids, and yes, a sinus infection IS contagious if any of her secretions (airborne or direct contact) came in contact with another child. It's just common courtesy... that's all.