“I have an uncle and cousin in Boston”.

 (Excerpt From The Blood of Olympus).

 “...a teenager wouldn’t be wrapped in a stinky old sleeping bag, stuck outside in the middle of a Boston winter”.

 “Annabeth. Which meant the sandy-haired man was … Uncle Frederick?”.

 “You’re lucky you live with your momma. Annabeth stacked another domino on......” {Annabeth's not live with her mother (Athena) and/or have problem with her stepmother}

 "Annabeth looked at me. Her grey eyes..."

 "Frederick and Annabeth still lived in Virginia..."



 ( Excerpt From Magnus Chase and The gods of Asgard: The Sword of Summer preview on Percy Jackson and the Greek Heroes)

"He had sandy-colored hair like Annabeth ..."

“Dr. Chase jumped up and started patting his pockets. “My keys…” His wife sighed. “Frederick, honestly. You'd lose ”

“Ah, well. You did leave quite a few half-blood weapons in your room in Virginia, the last time you… left.” Annabeth looked down, embarrassed. I noticed Dr. Chase was very careful not to say ran away.”

(Excerpt From The Titan's Curse)

Yep, apparently they're cousin, :)