Please register to participate in our discussions with 2 million other members - it's free and quick! Some forums can only be seen by registered members. After you create your account, you'll be able to customize options and access all our 15,000 new posts/day with fewer ads.
Well your mom's cousins's sister would also be her cousin, unless she's a half or step-sister, and not related to your mom, which means you're not related to the son at all.
Assuming everyone is blood related, and assuming by cousin, you mean a 1st cousin - your mom's first cousin's son is your 2nd cousin.