Does my bf have a good reason to not like his sister's "man"?

Asking out of curiosity for your thoughts.

My bf’s older sister just got out of a relationship of 4 years. She moved back in a few months ago. She is 27 and my bf is 23. She was sad but soon after her break up she began dating some guy she’s been friends with at her job. They’ve been seeing each other for 2–3 months now.
My bf talks passive aggressively about this guy and makes “mean” jokes about him when they aren’t around. Kind of gave her a hard time last night saying she needed to come home since she’s been staying at his place the last few days. He makes excuses why he doesn’t like the guy like “She sees him a lot, he’s supposed to pursue her”. or “She spends too much time with him.” His other sister agreed that it is too soon.

For crying out loud she’s the oldest and they are treating her like a child telling her what to do.

He never acted like this with his other sister or this sister’s past relationship. Honestly the guy is a lot nicer and comes to the family functions unlike her ex who never came around.

My bf said “she’s going to get hurt” and I told him you can get hurt no matter what and that I could even get hurt by him. I also mentioned its more likely she’s rebounding on the guy that really likes her and she’s using him to get over it so if its anyone to be hurt its the guy most likely.

The whole family is iffy about him because he’s a few years younger than her and she moved on quick.