Ariel Winter's Brother -- If Mom Can't Have Custody ... Give It To Me!

Ariel Winter's older brother has just filed legal docs asking to strip custody of the 15-year-old "Modern Family" actress from her sister ... and award it to him instead ... claiming the sis is an abysmal guardian.

Jimmy Workman -- who played Pugsley Addams in the 90s "Addams Family" movies -- filed the docs in L.A., calling Ariel's older sister Shanelle Gray a bad role model who sexualizes Ariel ... and exploits Ariel's fame and money to promote her acting school.

Jimmy says Shanelle has a history with drugs, has appeared in public "almost nude revealing her private parts," and is having Ariel spend exorbitant amounts of money on a party lifestyle.

What's worse ... Jimmy says Shanelle is poisoning Ariel against her family, especially her mother Chrystal Workman.

Since the court has refused to return custody to Chrystal -- after Ariel accused her of being emotionally abusive -- Jimmy is now asking for custody to be transferred to him.

Shanelle's lawyer Mike Kretzmer tells TMZ, "The allegations are nothing new. We will handle them in court. Shanelle has taken excellent care of Ariel to date. It's interesting how Jimmy and Chrystal talk about repairing their fractured family, and then make allegations like this."