These two adjectives are used to compare among persons in the same family.
Rahim is my elder brother.
She is my elder sister.
His brother is elder than him.
She is elder than her sister.
Who is the elder of the two?
My father is the eldest son of my grand-parents.
Janie is the eldest daughter of her parents.
I am the eldest son of my parents.
Who is your eldest brother?
These two adjectives are used to compare between two different family members.
David is older than Jane.
His sister is older than me.
I am not older than him.
Your brother is older than me.
The man is older than any other man in the village.
Shamim is the oldest man in this village.
She is the oldest woman in this area.
Your father is the oldest member of this mosque.
Dhaka is the oldest city of Bangladesh.
Her mother was the oldest woman in the village I have ever seen.