Advice Please!

We are going to Mexico in 5 days with the whole family including my 7 week old son for a dear friends wedding. Grandparents are coming with us and we are all flying and staying together so that we have extra help. I just found out that they are going down to see my sister in law tomorrow and she is sick. I have asked them not to go because I don’t want to potentially spread illness to my 7 week old but sister in law is really upset that we would even request them not to go and is causing a big stink about it. They don’t want to deal with her and just want to go see her and also go to Mexico with us too. But I feel like I am being put in a poor position because we are staying with them for a week and now potentially exposing us and baby to illnesses when we wanted to have a good trip without the stress of potentially having a baby sick in Mexico. I feel like they are being careless and I have expressed this to them, but not sure how else to move forward. Normally we would just avoid them if they were around a sick person but we can’t in this case and I’m very frustrated because I feel like my power of choice has been taken from me. What should I do?