Grandbrother
When your 90+ grandmother has conjugal visits in a nursing home and becomes miraculously pregnant, resulting in a baby boy
Dude, last night I had to babysit my grandbrother because my grandmother broke her hip.
great grandbrother
When your brother marries your great grandma
Here's an example: DON'T FUCKING DO IT. The Great Grandbrother is a disgusting thing.
Grandsister/Grandbrother
When your son/daughter has a baby with your mom/dad, said baby becomes the your grand-sister/grand-brother.
Today I'm taking my Grandsister/Grandbrother to the park.