Protecting My Sister or Overreacting?

AM I THE ASSHOLE? for smacking my stepdad after he hit on my younger sister?

I (16F) live with my mom, my stepdad, and my younger sister (11F).

My mom works late a lot, so most evenings it’s just us at home.

A few nights ago, my sister came to me crying and asked if I could “make him stop being weird.”

That immediately scared me. After some pushing, she told me that my stepdad had been

making comments about her appearance, standing too close to her, and telling her

not to tell anyone because he was “just joking.” I was furious but tried to stay calm.

I went to the living room and confronted him, asking what he had been saying to her.