"Modern Family" star Ariel Winter broke down in tears in court just moments ago ... after her mom played a schmaltzy home video of Ariel's childhood memories.

Chrystal Workman played the video for the courtroom as a weird last ditch effort to get her daughter to change her mind about living with her older sister Shanelle ... instead of with her mom.

Chrystal told the court she would relinquish her bid to reclaim her parental rights over Ariel -- after a judge stripped her of custody in 2012 -- if Ariel agreed to watch the video ... which she did.

The home video contained piano instrumentals and pictures from Ariel's childhood -- as a baby, from school, of the whole family together, and more.

Ariel, Shanelle, Chrystal, and several other members of the Workman family were in tears as the video played -- but Ariel didn't change her mind ... and the judge signed off on Shanelle's permanent guardianship.

As we reported, Ariel had accused her mom back in 2012 of emotionally and physically abusing her. Ariel's been living with her older sister Shanelle ever since.