Some mothers are a bit competitive that way.
My mother has a cousin who gave birth to her daughter on the same day that I was born...
There was always a bit of snark and competitiveness from the other mother - which I was aware of to an extent. As I got older, I became more aware of it, but was a bit bemused as to why my mother seemed to resent her cousin so much.
(She was perfectly polite to the woman to her face, but I could tell.)
My second cousin and I finished up at the same secondary school and I became aware that her attitude towards me was rather patronising, to say the least. You had to be there, but there was a lot of sniggering.
One day, she looked at me and giggled: "You know, Weary, people would never think that we were cousins!" In the end, I mentioned this to my mother.
"Humph! And what did she mean by that?"
"I'm not sure. I think because she's prettier than me because she's blonde and I'm not?"
"Humph! It's out of a bottle, the same as her mother's blonde hair!"
I really was bewildered. Mum then told me that when we were both at primary school - different schools - she had bumped into her cousin. Her cousin started going on about her daughter's achievements. Normal enough.
Then: "Plus my daughter is top of her class for arithmetic!"
My mum: "That's good. Weary's top of the class for arithmetic and English."
Pause. Mum's cousin: "Well! They do say that children of mixed blood do well at school!" [Dad was a wartime Displaced Person.]
I'll add that the other girl mellowed with age. Her mother - not so much.