Rob Reiner and Michele Reiner Singer‘s loved ones are relying on one another in the aftermath of their tragic deaths … gathering just one week after their tragic deaths.

Rob’s sister Annie‘s house was buzzing with activity Sunday … as Rob’s younger brother Lucas also arrived to the house.

Annie was spotted out on the street … speaking to someone in a car while wearing an all-black sweater and matching pants. Despite the difficult circumstances, she put on a brave face and fave a wave to guests.

Lucas parked in the driveway and walked up in a black beanie, suit jacket and slacks. He carried what appeared to be wine with him.

It appears someone brought desserts with them … though it’s unclear if this is an informal celebration or simply a family gathering to discuss the obviously unique circumstances surrounding the couple’s deaths.

As you know … Rob and Michele were found dead in their L.A. home last week after suffering knife wounds. Their son, Nick Reiner, has been arrested and charged with murder.

Sources with direct knowledge told us … Nick was diagnosed with schizophrenia prior to his parents’ deaths — though we’re told doctors hadn’t yet found the correct, stablizing dosage.

Rob and Michele’s other children, Jake and Romy Reiner, released a statement in the aftermath of their deaths which read, in part, “Words cannot even begin to describe the unimaginable pain we are experiencing every moment of the day.”