This is going to be long sorry in advanced but, could really use the help & I don’t want to leave anything important out. FTM here, we have a 9 WEEK old. He got his first round of vaccines 12/15 (thinking it would be  slightly more beneficial knowing how many people we were going to come in contact with for Xmas eve & Xmas). 
Usually me and my partner would go to my grandparents house and his parents house on Xmas day but that seemed like it would be overwhelming for us and the baby so we spilt it into 2 days. 
Me, my partner & the baby went to my grandparents house for Xmas eve and people stopped in and out, I texted my family group chat a few days prior to not stop buy if they themselves were sick or anyone in their household and also to not kiss the baby, especially because it was some people’s first time netting the baby so I wanted my wishes to be clear. Everyone replied and on the day obliged to my wishes with reassurance that I wasn’t being extra or anything, it was really comforting. 
The next day we all go to my partners parents house. I did NOT text them as they live much closer than my family & ALL have seen the baby multiple times now & know my wishes- I did not think they needed to be repeated this time. We were there for SEVERAL hours before we were informed that someone living with them had ‘just been getting over’ the flu that she’s had for 2 weeks & that she had been quarantined in their room the whole time we were there(which didn’t even happen she came out of her room multiple times for things that’s how it got brought up in the first place, we quietly both assumed she was elsewhere for the holidays). They claimed she was ‘no longer contagious’ & that they disinfected everything prior to us arriving. MIL is a CNA & has been pretty much her whole life. How does she not understand that everyone else (regardless if those last two statements were true) in the house could be a carrier of the flu still - even if she was ‘no longer contagious’??? She has aslo not come to our house when planned to because she herself has gotten sick or someone in her household. So I’m GENUINELY confused as to why we weren’t informed ahead of it THIS time??? She’s not a bad or crazy MIL, we have a decent relationship but we’re also not as close as I’ve been with other past MILs either. 
Yesterday we brought the baby to the hospital because he was extremely warm & lethargic with a fever of 100.7, just to find out he has the flu. Partner texted his mom when we found out & let her have it. He’s just as upset as I am about it all. But I’m actually so enraged though and trying not to overreact or break relationships over this. I am no contact with my mother (4+ yrs), don’t know my father. I don’t want to just write his parents off. I want them to understand their stupidity tbf but also in a nice but stern manner if that makes sense. I just don’t know how to handle this without blowing up, please help. If anything doesn’t make sense or I didn’t clarify enough I will gladly explain further 🙏

Mom guilt and PP rage taking over - need help replying in a considerate but firm way.