YTA, no question. Everyone saying you’re NTA is assuming the guy is a bad guy, or they think it’s wrong to date someone with an age gap. You went behind her back and outed her to your parents. Are you 12? Why is your first thought to tattletale to your parents without even speaking to your sister first? She’s a grown woman and can do whatever she wants. She could have a millions reasons as to why she’s keeping it a secret, but you wouldn’t know that because you went behind her back. Most people that I know that has dated older people were too ashamed to tell their parents because they thought they’d be judged. She should have the right to come to terms with her reasoning and tell your parents when she’s ready. You took that all away from her, and in a demeaning way at that.