いとこはcousinで男女関係なく同じ単語で表すことができます。
cousin（いとこ）
ex)She is not my sister. She is my cousin.
彼女は姉（妹）ではありません。いとこです。
一方で、姪や甥に関しては性別によって言い方が違ってきますので
注意が必要です。
niece 姪＝兄弟（姉妹）の娘
nephew 甥＝兄弟（姉妹）の息子
お役に立てれば幸いです。
The word "cousin" is for a child belonging to my aunt or uncle and is the same word whether it is a girl or boy. An aunt or uncle is my parent's sibling.
However, if my brother or sister have a child than that child is my "niece" if it is a girl and a "nephew" if it is a boy. It depends on if the child belongs to your sibling or your parent's sibling on what it is called.
Talking about your Aunt or Uncle, their children would be your Cousin or cousins.
Example sentences
" I have 5 cousins that live in Norway." - Talking about how many cousins you have.
"My aunt just had a baby! So I have a new cousin!" - Talking about the relationship, that the cousin is your aunt's child.
"Are you excited to see your cousins this summer?" - Talking about visiting or seeing your cousins/family relations
叔父または叔母の子どものことは "cousin"（単数形）あるいは "cousins"（複数形）といいます。
例文：
"I have 5 cousins that live in Norway."（ノルウェーにいとこが5人住んでいます）
- いとこの人数について。
"My aunt just had a baby! So I have a new cousin!"（叔母に子どもが生まれました。なので、私に新しいいとこができました）
- 関係性について。いとこは叔母の子どもです。
"Are you excited to see your cousins this summer?"（夏にいとこに会うのは楽しみですか）
- いとこに会う［会いに行く］ことについて。家族関係について。
Cousin: is the term that refers to either the son or daughter of your aunt or uncle
Cuz: is a slang term that can be used when you are talking to or writing to your cousin. E.g. "Hey cuz! What's up?!?"
A cousin is the son/daughter of your aunt/uncle we used the term 'cousin' for both male or female there is no different word
A niece is your sister/brother's daughter, a nephew is your brother/sister's son
Your aunt or uncles child is called your cousin. You can use any of the sentences above to talk about this. For example, 'he is my cousin' when you are talking to another person about your cousin. Or you can say 'we are cousins'.
If you are talking about two other people and are describing how they are related, you could say 'they are cousins'
叔母あるいは叔父の子どものことは 'cousin' といいます。
上記の文はどれもこれについて話すときに使えます。
例えば、'He is my cousin'（彼は私のいとこです）は、誰かに自分のいとこについて伝えるときに使えます。もしくは 'We are cousins'（私たちはいとこです）とも言えます。
その場にいない二人の人についてなら、'They are cousins'（彼らはいとこです）と言えます。
日本 
アメリカ合衆国 
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