Step brother won’t get a flu shot.

My step brother is 10. He will not get a flu shot because he doesn’t like shots so he refuses. What makes things worse is my step mother is a nurse and you would think she’d make her kid suck it up and get a shot...... Anyways, I know the flu is not usually around in July, but my entire family has gotten the flu in july a while ago. So i don’t want my newborn around him if he doesn’t get his shot.... I’ve tried to set that boundary before by saying no one gets to be around baby unless they’ve had shots and my step mother didn’t say a word and still hasn’t taken her child to get a shot and this was back in December.... What should i do? I dont even know if i’ll be letting people around my newborn anyway with Covid going on but if things are ok by July then I don’t know what to do.

EDIT:

I know just because you get a flu shot doesn’t mean you’re completely protected from the flu. Someone close to me died last year from the flu unexpectedly and she was otherwise healthy so we take the flu and the vaccine very seriously. I made the post asking for advice on what to do, didn’t ask anyone’s opinion on if the flu shot works or not.