Sister in law putting MIL at risk.

My SIL is pretty lazy and her husband works out of town a lot. She takes her two younger kids over to my MIL’s house and essentially moves in for the week or two while her husband is gone because she doesn’t want the kids by herself.My MIL has severely damaged lungs from almost losing her life from the flu several years ago and she is suuuuper high risk right now. A few weeks ago one of her kids came home from daycare with a fever. She immediately packed up their stuff and went to stay with my MIL because she didn’t want a sick baby by herself. I was so upset and was livid that she would put her at risk like that. I expressed my concern and she was like, “oh he’s fine, it’s just a little bug”. Fast forward to this week and she calls yesterday and said her hubby has a super high fever, feels awful and tested positive for antibodies. The doctor told them to quarantine for a week and get another test. She immediately calls my MIL and asks her if she wants her to stay home or if they can come stay with her. I said no, She needs to stay home. She called me overbearing and said it’s none of my business. Did I overstep? I’m also a nurse so I take COVID seriously but their town is having a huge surge is cases. My MIL is a huge pushover and said she could never say no to her grandkids. I’m more upset at my SIL for even putting her in that position.