three-quarter-sibling
Child of a person's father with that same person's maternal aunt or child of a person's mother with that same person's paternal uncle.
My three-quarter-sibling is a good person.
Three-Quarter Siblings
When a man and a woman have a baby together, then the woman realized she chose the wrong spouse when she met the man’s brother. They decide to have a baby together. These two children are known as “three-quarter siblings.”
“Dude! I heard she fucked his brother, and had ANOTHER kid! They are going to be three-quarter siblings!”