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This question already has an answer here:

Why in this sentence "older " is the correct word?

He married the older sister and now they have two sets of twins.

He married the elder sister and now they have two sets of twins.

marked as duplicate by user070221, Andrew, Nathan Tuggy, Jason Bassford Supports Monica, Varun Nair Nov 22 '18 at 12:47

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'Older' is an adjective.

'Elder' is a noun, which when compounded with another noun can look like an adjective (with the same meaning as 'older').

Both of your example sentences are grammatical.

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