She is_____than her brother.

Ans: 2 (She is older than her brother) Because:

Older is the comparative form of the adjective old.

Elder is simply an adjective.

Ans: 2 (She is older than her brother)
Because: Older is the comparative form of the adjective old.

He is old.

He is an old man.

He is older than Methuselah.

Elder is simply an adjective.

He is Methuselah's elder brother.

Elder is used with a sense of comparison but it doesn’t use a comparative structure. We don’t say ‘*elder than…’.

‘Elder’ uses the same grammar construction as a regular adjective.

He is John’s older brother.

He is John’s elder brother.

It can also be used as a superlative in both constructions.

He is John’s oldest brother. He is the oldest brother.

He is John’s eldest brother. He is the eldest brother.

When we are comparing people’s age, we tend to use old-older-oldest.

When we are comparing family members we tend to use elder-eldest.