As an older brother to a sister 4 years younger than me, I used to be very protective of her. I dunno, it was something I felt I had to do because I loved her and I was her oldest sibling, and she looked up to me growing up. Same with my little bro. This was especially true in times of foster care and such.
Nowadays I am not like that at all, seeing as all 3 of us are legal to drink and are technically adults who should be mature (I know I get silly on the internet, but IRL I am very mature, mostly). My siblings are smart and can fend for themselves because at a certain point I stopped being protective of them. They gotta fend for themselves at some point.
In essence, I was like this until maybe 17 years old. I did it because I loved my dear little sis to the point that I didn't want her to be hurt emotionally or whatever.