I'm offended

First off let me start by saying that I'm married and my husband has been a father figure to my 5 year old son since he was a baby. My 5 year old is laying in bed tonight and asked where his brother's dad is (my husband) then commences to tell me that his dad is dead so he don't have a dad and that his father was killed. I never gave him this info so I'm surprised to say the least. Now keep in mind, his father was brutally murdered while I was pregnant with him. My sister had him on vacation for the past 2 weeks and got upset when she found out my 5 year old calls my husband dad. I truly suspect that she or her sons told my 5 year old this bit. I'm angry. Am I wrong?

Update: My sister admitted to telling him. I'm heartbroken 💔 we are going to trial this month. Praying for justice. 🙏