Is it wrong for a 19 year old boy to hit on a 13 year old girl?

Yes, it is wrong. Quite wrong. You are not overreacting at all, and your parents will thank you for it (provided your parents are level-headed, rational people, as I assume they are). The age of consent varies state by state, but there is no state with an age of consent lower than 16 years old. Please do your sister and yourself a favor and talk with your parents about him as soon as possible.