My brother has some of my genes.

We share some of the same genes due to our common parentage.

Therefore, helping my brother get laid will help pass my genes forward, which is the meaning of life.

Same with your sister. You should be inviting Chads over to creampie your sister.

You won't because spreading genes by proxy isn't your actual motivation. You can insert yourself as your brother, so his success would validate you and give you hope. In the end it's still selfish.