My mum and grandma say: “If a girl sits with her legs apart that means she is not a virgin anymore.” Is that true?

You cannot tell something like that by the way a person sits.
That is something older generations used to believe years ago.
Because they wanted girls to sit with their legs together.

There are lots of similar stories (myths):
For example: “if girls ride bicycles they are no longer virgins.”
Or: “girls shouldn’t jump as it will tear their hymen.”

These sayings can be confusing for girls.
They restrict girls in their movement or behaviour.
Some girls become scared to behave in certain ways.

There are similar opinions about periods and pregnancy:
For example: “if a woman has her period she should not wash as she could become ill.”
Or: “women can get pregnant in the swimming pool without having sex.”

These sayings are not correct.
If you are a girl it is best to ask your gynaecologist.
She can provide you with the correct information about this topic.