Not finding it funny

I am really pissed because I heard my mum telling my 7 months old daughter to call her mummy so when I asked her if my daughter calls her mummy then what will she call me and she replied sister gift.

CONVERSATION BTW MY MUM AND I

Me: Mum please stop telling my daughter to call you mummy.

Mum: Why do you say that?

Me: Because you are her grandma and not her mummy.

Mum: Well I want her to call me mummy.

Me: So if she calls you mummy rather than grandma then what will she call me her mother?

Mum: She will call you sister gift.

Me: Why will I want my daughter to call me sister when I am her mum.

Mum: I just told you what I want.

Me: That’s not gonna happen because she’s my child and not my sister.

At this point, I will have to limit our visit to her and I don’t care if she’s gonna feel hurt because she is crossing her boundaries.