Your sister's child is your nephew; Children are cousins, cousins, cousins.

Cousin means that your grandfather and cousin's grandfather are brothers; And uncles and brothers say your father is a brother. Cousins are also paternal. For example, if you have an aunt, her son and you are cousins, usually called cousins for short. And your mother has sisters, that is, your aunts, whose children are cousins with you. That is, cousins, it is precisely in the title that "aunt" cannot be omitted to distinguish cousins.

I hope my answer can give some reference to the landlord.

If you ask children what to call each other. That's easy. I'm still called cousin sister. I feel very close, and I don't say much. Cousins, too, have nicknames.