Patrick Muldoon's Sister Posts Final Video He Sent Her

Patrick Muldoon showed off his humor and energy in the last video he ever sent his sister ... even pulling out his old high school chant in the heartbreaking clip.

The actor's sister, Shana Muldoon Zappa, shared the video to Instagram late Wednesday night ... revealing her brother sent it on Saturday during a preview opening of the new Los Angeles County Museum of Art.

Check out the clip ... Muldoon excitedly films a painting of St. Ignatius of Loyola -- before breaking out into a chant.

Zappa explains her bro was a fan of the saint ... and, he went to Loyola High School -- so, the chant is a nod to his football-playing days way back when.

She remembers her brother as "the joke-ster, the artist, the football player, and the intensely spiritually connected, Jesuit-educated, incredible being."

Zappa adds the family will eventually hold a service ... but, they're still in the middle of processing their grief -- and, they haven't planned anything yet.

As you know ... Muldoon was pronounced dead after his girlfriend found him unconscious in the shower Sunday. His last few Instagram posts showed no signs of distress -- instead showing a man just living his life before his tragic passing.

Patrick was 57.