She feels that since she had it, if the others were going to get it they would have gotten it already from her and are therefore supposedly immune? Do you know if they have been out and about? Or are they all living in their house together and never go to the store or anywhere?
Sometimes we just have to do what seems like the correct course of action, even if we have only "I feel that way" to defend our position. She might be right but you might be right, and it's your choice to go or not.
Just say that you don't feel safe going to gatherings. That takes it out of being a judgement on her ability to assess the situation, and puts it all on you. She is not as likely to feel insulted that you haven't heeded her judgement if you posit it as merely your own fears.
As my old friend the therapist always said at Christmas, despite the heavy expectations of the season, we are not *obligated* to go to gatherings if it's too much. That was true back in the days when "too much" was merely referring to the overwhelm of the season, and it is true today with Covid. Do what makes you feel the most protected.