Were Abraham's relatives in Syria also idolaters?
Yes. Everyone in the then-known world from Babel onward was idolatrous until Abram was called out; apart from Abram, Isaac, and Jacob, all were idolaters—including the relatives in Syria. Laban's concern for his stolen family gods or idols confirms this.
Les teaches that from the Tower of Babel onward everyone in the then-known world was an idolater until God called Abram out of idolatry. Abram, Isaac, and Jacob worship the one true God, but other than them, all were idolaters, including the relatives up in Syria. Les points to the account in chapter 31, verse 30, where Laban catches up with Jacob and asks why he stole his gods—small g-o-d-s, meaning idols. Les explains Laban's concern: these were the family gods, and by pagan custom the most favored child received them; having the family gods meant having everything.