We are currently visiting my in-laws and are 2 weeks into it. 
Anyway, we see my nephew most days. He is two. Since we have been visiting he hasn't called me, "Cas" or "Aunty Cas," instead he calls me "Mum." The only reason he does this is because he hears my DS call me, "Mum."
Now, I do correct him when he calls me "Mum" but he is two and is obviously confused. He naturally calls his own Mum, Mummy. So that isn't an issue.
Yesterday however, I overheard my SIL asking my 6 year old DS to call me "Aunty Cas" when around my nephew. WHAT? I understand she probably feels sensitive about this but I will not have my son calling my "Aunty Cas." Especially because we see my nephew every single day while down here visiting.
So, I am thinking that perhaps a quick chat to my SIL is needed? Just to explain that I understand where she is coming from, but think that another action is needed when correcting what he calls me. I mean, he is only two... I am pretty sure he will grow out of this over the next few weeks/months anyway.
Before I speak to her though, I thought that obviously the world of MN needed to tell me whether I was being UR or not? 