KINGSTON, N.Y. (AP) -- The sister of a sex worker whose disappearance led to the discovery of 10 sets of human remains strewn near a New York beach has pleaded not guilty to stabbing their mother to death.

Prosecutors in upstate New York say Sarra Gilbert repeatedly stabbed 52-year-old Mari Gilbert last month.

It happened inside an apartment in Ellenville, in the Catskills. She was arraigned on murder charges Friday.

Attorney John Ray says Sarra Gilbert suffers from serious mental disabilities.

Her sister Shannan Gilbert vanished in May 2010 after fleeing a client in Oak Beach. The search for the missing woman led to the discovery of 10 sets of human remains on Long Island. Police believe the 10 were victims of a serial killer but Shannan Gilbert accidentally drowned.