Why can’t he see me as a sister?

There’s this guys who’s 1 year younger than me , the other day we were with our friend group and he said to us he has a sister and her name is X , another guy said :” no , your sister’s name is “ my name””

and his answer was “ 🤣 no I have “ my name” here and I can’t imagine having her at home and as a sister “

he was obviously joking about it , but why can’t he see me as a sister for him

I always think of him as a younger brother for me

Why can’t he see me as a sister?
Post Opinion