please use this link and scroll down a bit to my comment about ezreal and lux
https://boards.na.leagueoflegends.com/en/c/story-art/75EpeEiF-relationship-in-league
i tried my best to be informative about it.
also if you read garen lore you see that he loves his sister yet he cant get that close to her.so i would say no he is not a overprotective brother.
"Though Garen's sister, Lux, had followed in her brother’s footsteps, coming to serve the crown in Demacia’s capital, their relationship remained distanced. Garen refused to acknowledge the things he’d suspected of her before he left to train with the Vanguard. Though he always loved his sister, some small part of him had a hard time getting close to her. He tried not to think about what his duty would force him to do if his suspicions were ever confirmed."