My sister is a total bitch (16, boy). And I tell her so. Now she has told our guardian. He finds my language ‘sexist’. He’s mad, right?

The translation of bitch is a ‘female dog on heat’.
Unfortunately, this expression is sometimes also used for girls.
People often use it to say that a girl has too many boyfriends.
Or that a girl has already had sex.
It is a very offensive word for girls.
Your guardian is right. Words like this are sexist.
There are lots of other harmful words for girls.
Boys can also be hurt by works.
For example, with words such as ‘wimp’.

It does not matter which expression is used.
If it harms the other person it is not right.
It is good to talk to people directly.
It is not good to hurt them with expressions.
So you should apologise.

If you think your sister’s behaviour is not nice then talk to her.
It is important to show respect towards each other.
This includes the language you use: it shows that you respect somebody.