So basically a variation on the Drift AU, where Dipper cannot contact Mabel, despite how hard he tries. Instead he ends up following his sister around like an usually powerful ghost. With Mabel unable to see, hear, or otherwise sense him.
He is there is observe Mabel and Henry’s awkward courtship (Henry had the strangest nightmares for weeks after meeting Mabel) and he’s there to see the Triplets being born.
While Mabel can’t see him, the Triplets can. It is one of the most profound experiences for Dipper when the triplets look his way and see him, and again when he holds out a finger and they reach up to grasp it. He then pronounces their true names and gives them each his mark. Mabel and Henry see nothing.
The triplets then grow up with an guardian demon/invisible uncle looking out for them. Mabel for her part is sort of relieved that her bedtime stories about her brother have made an impact on her children, especially when they start having an invisible friend they name Uncle Dipper.
Then one day, the Triplets get snatched by a cult looking for a sacrifice. Mabel grabs her bat and charges off after them.
The cultists for their part, realize they are in serious shit when the mid-level demon they summoned gets devoured before their eyes as an absolutely livid Alcor the Dreambender forces his way through their summoning circle. (You DARE!)
Mabel breaks down the door just in time to see a demon that looks almost exactly like her long dead brother, whom her children call “Uncle Dipper”