AITA for giving my sister boyfriend advice?

My sister (14f) have a boyfriend (14m), they look pretty good together but I personally don't like him, at the same time I don't go out of my way to say or do anything about him. About 2 months ago my sister have been asking me for advice because as she stated "your a guy so you would understand". She would tell me incidents where her boyfriend would do certain things that she didn't like, so she came to me and ask me if that's how guys usually act. 90% of the time I would tell her just talk to him and tell him how you feel. Now here's were the problem begin, my sister and her boyfriend went for a walk at the neighborhood park (they had masks on). He saw a girl with tights on going for a run and he started starring at her, my sister noticed but didn't say anything, although she felt uncomfortable in this situation but she didn't want to make a big deal about it.

This is the other 10%. So she came up to me and she said "for awhile now my boyfriend have been starring at other girls more and more. Before he would just give other girls some glances but now his just full on starring. I feel like he finds other girls more attractive then me, what do you think I should do?". I just tell her what I usually tell her just tell him how you feel, she said he did that but he just keeps doing it. This Time I told her "usually when a guy starres at a girl maybe they find them pretty but as long as they don't flirt with the girl they're clear. If she feels like his cheating on her she should ask his friends or brothers if they noticed him talking to other females lately? Maybe she might find something"

A week later or last Saturday she broke up with him because she caught him cheating. She told him that I gave her to advice to find out if his cheating, and his saying because I never liked him I make my sister break-up with him. I told my friends about this some of my friends said I should've never got involved because "it's not my relationship".

So AITA for giving advice to my sister?