The 80s cartoon never really got too deep into Skeletor's origins, and never introduced a brother for Randor. Post-cartoon, late in the toyline, there was a minicomic about Skeletor trying to stop Randor from learning what happened to his brother Keldor, and it comes across like maybe Skeletor IS Keldor, but the story never gets around to just saying that.
As I recall, the 2002 cartoon established Skeletor was originally a man named Keldor, but never got around to revealing Keldor was Randor's brother. That was a reboot, so it's debatable whether you can use that 80s minicomic as evidence.
Years later, I think a toy pacakage bio finally connected all the dots so Skeletor is Keldor AND Keldor is Randor's brother. But that was yet another separate continuity, cherry-picking elements from the earlier stories. And the movie isn't strictly beholden to any of those stories.
So I'd say it's a 50-50 chance whether Travis Knight means to reveal Skeletor is He-Man's uncle someday, or if he thinks that's dumb and doesn't want to do it. And we don't even know if the next movie will actually happen, let alone if it'll be a true sequel or a reboot.
Most of the unresolved questions in Masters of the Universe canon are like this, so I wouldn't worry much about it. Unless you're shipping them and want to avoid a "we put Luke and Leia on our wedding cake in 1979!" predicament.