Well that confirms that Tsuyu and the sister are also around 14. Considering the brother just got his license, that means he is at least 18. He could be 16 if it was a moped or motorcycle, but then he wouldn't have been allowed to give two younger girls a ride.
...To be honest, a 4 year gap is not that big. I mean yeah, don't date her NOW, but it's a pretty reasonable gap down the road. Especially considering how other series would set the gap.