In-laws want to secretly show up sick to gender reveal party

Me (25f) and my husband (24m) have been planning our gender reveal for a few weeks, making sure our house was clean and ready. We held it on the 28th, right after Christmas so that my brother could be there for it in person. We invited 24 people in total- 9 of which were in-laws.

On the morning of, we get a call from my teenage SIL saying she was recently sick with a fever and wouldn't be able to join us. Concerned, we reached out to my MIL, only to find out that her, her children, and grandchildren had all been passing some kind of illness around over Christmas. She didn't call to warn us. I would have just rescheduled the entire party. She genuinely wasn't going to tell either of us that they still had various symptoms and were possibly contagious. All 9 of them.

She cried when my husband told her she couldn't go. Not only do I not want to risk getting the flu or something worse, but I don't want to risk it for my guests or their children either.

Not much else to say about it. Just extremely frustrating. Who does this shit?