Am I overreacting with this matter?

I helped my sister get an event ready at a banquet hall. My nephews were there to help. Our acquaintance showed up to help and kept following my nephews around and was playfully joking around with my sister. My sister is not the biggest fan of this particular acquaintance. I saw their interaction and was not impressed. My sister saw my behavior and asked if I was Okay. I said I was just tired. Am I overreacting? Am I just being protective?