Well, if your brother lives near Nyack, then Pearl River isn't far. Knitting Nation is the Nyack store, and it's on Broadway. (Main Street is the one that goes down towards the river, Broadway is the street that intersects it by the Starbucks.)
The Pearl River store is called the Stitchery, on Central Ave. Like I said, I've never been there, but to get there from Nyack, you would take Main Street, which turns into Route 59. Take Route 59 to 304 South, which goes right through Pearl River.
KnottyDaisy has been there and she said it's like floor to ceiling yarns.
I just googled and found this
list of New York State yarn stores and this
list of New Jersey stores. I'd call ahead of time to make sure they're still there--the Englewood store listed on the NJ list moved to Vermont.