Please tell me I’m not over reacting!!!

So I’m not sure where I should post this so I just posted it here.

Here’s a little back story before I get into the issue. When I was 19 I got pregnant by my boyfriend at the time he. We ended our relationship before our son turned 1 he had anger issues threatened to beat my ass and also laid hands on me. He committed suicide in 2014. He family still has contact with my son and he goes and spends the summer with them.

Now here is where the issue starts; I got a phone call from his grandma concerned about an issue she was having with him for a couple weeks ( he was diagnosed with adhd last November ). While she is rattling on she then proceeds to tell me that he calls my SO dad and sometimes by his name which I knew. BUT the part I’m having issue with is the fact that her 29 year old pregnant daughter over heard him call my SO dad and took it upon her self to tell my son ( 7year old) that he only has one dad and he is in heaven that my SO is his step dad. That he shouldn’t be calling him dad. I get the fact that she was close to her brother. But come on pregnant or not you don’t tell a child someone else’s child at that not to be calling someone else dad. I see no issue with him calling my SO dad, he is aware that his dad was sick didn’t take his medicine and passed away. He has told me multiple times that he likes having two dads one in heaven and my SO. I’m just having a hard time swallowing this down and keeping my mouth shut. Apart of me wants to message her and tell her she has no right filling my child’s head with things and I also thought about not letting him go back next summer. I don’t want this to be a continuous this.