Having a high-achieving older sibling can help children do well in school, according to a new study.

The gender of a person's siblings doesn't appear to have a meaningful effect on their personality traits, research in nine countries finds.

Though it can be a tough thing to admit, parents really do have favorite kids. But their adult children are usually wrong about which sibling mom favors.

Astronaut Scott Kelly spent 340 days in space. His twin brother, astronaut Mark Kelly, stayed on Earth. Here's how their mitochondria compare.