There is no difference between elder/eldest and older/oldest.
Normally, we use older/oldest with general things like;
"I think her grandfather must be older than her grandmother."
"The town hall is by far the oldest building in the whole region."
"John is the oldest of our children and Mark is the youngest.
Normally we use elder/eldest before a noun.
"She's my elder sister."
"I am the eldest of four brothers."
"It's my fiftieth birthday on Thursday and I am now the eldest member of the family."
"elder/eldest" と "older/oldest" には違いはありません。
普通、一般的な事柄には "older/oldest" が使われます。
"I think her grandfather must be older than her grandmother."（彼女のおばあさんは彼女のおじいさんよりも[年齢](https://eikaiwa.dmm.com/uknow/questions/33031/)が上だと思う）
"The town hall is by far the oldest building in the whole region."（市役所は間違いなくこの地域全体で最も古い建物です）
"John is the oldest of our children and Mark is the youngest.
（子どもたちの中で[一番](https://eikaiwa.dmm.com/uknow/questions/66588/)年齢が上なのがジョンで、一番下がマークです）
普通 "elder/eldest" は名詞の前に置かれます。
"She's my elder sister."（彼女は私の[姉](https://eikaiwa.dmm.com/uknow/questions/38531/)です）
"I am the eldest of four brothers."（私は4人兄弟の一番上です）
"It's my fiftieth birthday on Thursday and I am now the eldest member of the family."（木曜日は私の50歳の誕生日です。私は今家族の中で年齢が一番上です）
"Sibling" is an easy way to say "brother(s) and sister(s)" in English since we don't have gender in our grammar. If you say that you are the oldest sibling in your family, all native speakers will easily understand what you mean. I hope that this helps! :)
"sibling" は "brother(s) and sister(s)" の簡単な言い方です、"sibling" には性別がありません。
相手がネイティブスピーカーなら、
"I'm the oldest sibling in my family"（私はきょうだいの中で一番年上です）
のように言えば、すぐに意味が伝わります。
参考になれば幸いです！
My parents have other children, but I am the oldest son.
It depends on if you mean you are the oldest of the male children or oldest of all of the children such as first born.
Hello, I am Johnny. I am their oldest son.
I have an older sister but I am the oldest son.
My parents have other children, but I am the oldest son.
My parents have other children, but I am the oldest one.
I'm the first born in the family.
これは、男の兄弟の中で最も年齢が上なのか、それとも全ての兄弟の中でなのかによって言い方が変わります。
Hello, I am Johnny. I am their oldest son.
（こんにちは、ジョニーといいます。私は長男です）
I have an older sister but I am the oldest son.
（私には姉がいますが、長男です）
My parents have other children, but I am the oldest son.
（両親にはほかにも子どもはいますが、私が長男です）
My parents have other children, but I am the oldest one.
（両親にはほかにも子どもはいますが、私が一番上です）
I'm the first born in the family.
（私は一番上の子どもです）
I'm the eldest son is a way to tell people that you were the first boy to be born in your family or that you are oldest of all the boys in your family.
"I'm the eldest son" は、最初に生まれた男の子、兄弟のうち一番年齢が上の者をいいます。
When you want explain that you were the first male born to your parents; then you may use the following phrases:
-Oldest son
-I am my parents' oldest son
Alternatively; you may explain by saying:
-My sister is my parents' first child. I am the second but the oldest son. I have three younger brothers that come after me.
自分が長男であることを説明したいなら次のフレーズが使えます。
-Oldest son（長男）
-I am my parents' oldest son（私は長男です）
あるいは、次のように説明することもできます。
-My sister is my parents' first child. I am the second but the oldest son. I have three younger brothers that come after me.
（私の姉は両親にとって最初の子どもでした。私は二人目ですが長男です。私には三人の弟がいます）
"Oldest son"
"I am the oldest son in my family"
This is stating that you in fact are the oldest out of your brothers and sisters.
"Oldest sibling"
'Sibling' is another term used when referencing your brothers and sisters, normally a more formal way of expressing this it is still widely used in a casual sense, ex. "My siblings and I will clean the house today".
"Oldest son"（長男）
"I am the oldest son in my family"（私は長男です）
これは、兄弟姉妹の中で自分が一番年齢が上だと伝えています。
"Oldest sibling"
'Sibling' も兄弟姉妹を指しますが、これはよりフォーマルな言葉です。ただ、カジュアルな場面でも広く使われています。
例えば：
"My siblings and I will clean the house today"
（今日きょうだいで家の掃除をします）
アメリカ合衆国 
南アフリカ 
イギリス 
