Have you seen Casper the Ghost? If not, I recommend it. It's a great movie!
Here you have a short review from this 1995 movie: James Harvey is a ghost doctor hired by Carrigan Crittendon to rid Whipstaff Manor of spirits so she can find a treasure trove rumored to be hidden there. But her plan backfires when James's daughter Kat befriends Casper, the friendly phantom who inhabits the place along with The Ghostly Trio (his "scary family"), who disdain humans.
The Ghostly Trio evolved from a group of housemates Casper lived with. Casper lived in a haunted house occupied by many ghosts, who, like in the paramount cartoons, scared people without Casper, but soon the house got smaller, and the number of housemates shrunk to four generic ghosts. When Alfred Harvey acquired the rights to Casper and other paramount cartoon characters, he settled on 3 older housemates for Casper, loosely based on the 3 older ghosts from the paramount Casper short "Fright From Wrong" where a trio of ghosts led by a fat one, who appear to be Casper's caretakers, force Casper to take mean pills to make him more ghost-like, but the plan backfires with hilarious consequences.
Fatso was the only distinguishable one for many years in the comic until the other two eventually got personalities and names, Fusso, the mid-sized fussbudget, and Lazo, the tall lazy one.
The comic never made it clear if the Trio were housemates or relatives.
In one story when Casper and Wendy run away because they don't feel like they fit in in their households, the Trio get worried sick and frantically search for him, while the Witch Sisters, who are Wendy's aunts (not by blood but they did raise her) do the same for her. When they find them the Trio apologize to him and behave like the Witch Sisters toward their young charge.
In another, Casper wishes on a magic balloon for all his best friends to be in one place and the Trio are among the first there. I'm not sure if they're uncles or his friends, but he's never called them anything besides 'The Trio' or by name.
Whatever the case, the Trio are in charge of the house and treat Casper like a slave sometimes which is more like older brother or big bully behavior, yet they care deeply for him (but not for his friendliness)
In the 1995 Casper movie the Trio were given a makeover to make them more interesting, and given ghostly "powers". Lazo was changed to Stretch and Fusso to Stinky. Fatso kept his name but was made less intelligent so Stretch became the leader, and they were considered Casper's uncles. Again, it's not clear if they're his guardians or relatives, but they are older and in charge of the house. Only Casper is given a backstory as a dead child, his "uncles" are never explained, like you said. They're either the dead brothers of one of his parents or just three ghosts who moved into his house and take care of him. Just like in the comic, they torture Casper yet treat him like a nephew, and Casper respects them as uncles. But Enough of this, If Casper says they're his uncles then they're his uncles and that's good enough for us.
Every movie and tv series that followed deviated from the movie story, but in 'Casper's Scare School' they're clearly his uncles.
In the 'Casper and the Spectrals' comic the Trio are given their original names back but retain their "movie look" and are outright Casper's uncles.
Hope you enjoy this blockbuster!