eldest child/brother/daughter, etc.
B1
the oldest of three or more children, brothers, etc., especially within a family
Dictionary examples:
The eldest sister always gets the biggest slice of pizza! (0.0)
As the eldest, I'm the family's official 'Wake up, it's morning!' alarm. (0.0)
Being the eldest means you're the family's unofficial tech support. (0.0)
Her eldest child is nearly 14. (17.8)