Inbreed
To inbreed means to have children with a member of your own family, specifically a blood relative.

This kind of thing happened centuries ago quite commonly to keep royal and high-class blood from branching out to the lower classes of society, and very rarely happens today, as it is frowned upon by...well, everyone.
"My dad forced me to inbreed with my cousin because he didnt want our family's blood going to some street rat."

"Im inbred."
"Lol."

