Would it be petty to tell my boyfriends family/have him tell them?

Going on anon because I’m worried I sound like one of those covid-obsessed people lol.

For thanksgiving my boyfriend’s grandmother and aunt (who live together - grandma is high risk bc of her age) want to do a small dinner with the family. The only ones invited are my boyfriend’s dad, my boyfriend, and I (we live together - his dad is high risk bc he has a chance of having/getting cancer and I’m pregnant) and my boyfriend’s brother, brother’s girlfriend, and their child.

At first, I felt safe because I thought everyone was staying home, especially because the brother, girlfriend, and child weren’t coming because of family issues. (Not my business) and The grandmother is retired, my boyfriends dad and aunt work from home. My boyfriend has been very anal about staying safe because of the baby and I’ve stayed home (I do work but laid off). But they (I guess? 🤷🏼‍♀️) resolved issues and invited the three of them.

Now, I’m worried because there are high risk people and I KNOW that his brother’s girlfriend is not taking covid serious. A little while ago, she took a weekend to go out of state. And she just told me tonight that she was out partying. I am contemplating telling the family that she is a risk factor because I know that if it were me in one of their positions, I would feel hurt that I wasn’t given a warning. I would feel guilty if I didn’t tell them and they got together with her and got sick. I want to try to protect them because it’s not fair to them. I don’t know what to do.