“Elder” and “older”, as comparatives, mean exactly the same thing.
Don’t get it confused with “elderly”, because that means “old”.
Once a man from Europe made a hilarious mistake in front of me. He was very charmed and excited by a young woman he’d met, but he thought she might be a little too young for him. So he said to me, “I wonder if she has an elderly sister!” He meant an “older sister” or an “elder sister”, but what he really said was that he wondered if she had a sister who was past retirement age.