「姉の娘の子供」は英語では、
"my sister's daughter's child"
または
"the child of my sister's daughter"
と言えます。
"sister"は「姉」または「妹」(英語ではそれらを区分しない)で、"daughter"は「娘」という意味です。あと、"child"は「子供」に相当します。
参考になれば幸いです。
By using the word "niece "your listener should understand the relation of the person whom you are talking about. You can change kid for son (if the child is male) or daughter (if the child is female).
I hope that this helps! :)
There are quite a few ways to say this. Here are a few expressions to use.
"My older sister's grandchild."
"My nieces child"
"My nieces son/daughter".
"My great-niece"
"My great-nephew"
Example :
My niece's daughter is so cute.
I love my great-nephew very much.
My great-niece is very naughty!
Nephew - boy
Niece - girl
All of those expressions are natural.
I hope it helps.
これはいろいろな言い方があります。
例えば：
"My older sister's grandchild."（姉の孫）
"My nieces child"（めいの子）
"My nieces son/daughter".（めいの息子／娘）
"My great-niece"（めい／おいの娘）
"My great-nephew"（めい／おいの息子）
例：
My niece's daughter is so cute.（めいの娘がすごくかわいいです）
I love my great-nephew very much.（めいの息子がすごくかわいいです）
My great-niece is very naughty!（めいの娘はすごくいたずらっ子です）
Nephew - 男の子
Niece - 女の子
これらはどれも自然な表現です。
参考になれば幸いです。
A child of your sister's daughter is your niece's child.
For example, you can say:
-My niece's child is 2 years old today. She has a birthday party today
姪の子供のことは、"niece's child" と言います。
（例文）
My niece's child is 2 years old today. She has a birthday party today.
（今日は、姪の子供の２歳の誕生日なんだ。今日、誕生日会がある。）
My sister's relationship with me is the same as that of my two brothers. In certain cultures, since this child is my sister's grandchild, I would refer to him/her as my grandchild too, or, more specifically, my sister's grandchild. It really depends on the culture being followed. But, since this is my sister's grand child, I would definitely refer to him/her as a grandchild by virtue of my relationship with my sister.
So, if asked about what you call your niece's child, you would say:
My sister's daughter's child is my grandchild.
or
My niece's child is my grandchild.
or
My sister's grandchild is also my grandchild.
私と私の姉の関係は私と私の二人の兄弟の関係と同じです。文化によるとは思いますが、この子は私の姉の孫なわけですから、私なら "my grandchild"（私の孫）あるいはより明確に "my sister's grandchild"（私の姉の孫）と言います。これは本当に文化によります。ただ、私ならこれは私の姉の孫なので、"grandchild"（孫）と呼びます。
めいっ子の子どもをどう呼ぶか聞かれたときには次のように言えます。
My sister's daughter's child is my grandchild.
（私の姉の娘の子どもは私の "grandchild" です）
My niece's child is my grandchild.
（私のめいっ子の子どもは私の "grandchild" です）
My sister's grandchild is also my grandchild.
（私の姉の "grandchild" は私の "grandchild" でもあります）
Your sister's daughter's is known as your niece.
Your sister's daughter's child is known as your niece's child.
Therefore your older sister's daughter's child will be known as your niece's child.
ブルガリア 
日本 
アメリカ合衆国 
南アフリカ 
