If we're talking about animation, my guess is either:  Thunderbirds 2086 (Japanese title Scientific Rescue Team Technoboyager)

Three (or maybe more) people (although not siblings) pilot individual craft that can merge into one.

Or

Battle of the Planets (Japanese title Science Ninja Team Gatchaman)

A group of five people (I think all siblings) pilot individual craft that can merg into one.

If we're talking about animation, then this could be: 

Thunderbirds 2086 (Japanese title Scientific Rescue Team Technoboyager)

Three (or maybe more) people (although not siblings) pilot individual craft that can merge into one.