Well, originally (this is according to the minicomics & Filmation "Bible"), Skeletor was a planet hopper, conquering planets one at a time with his magic and...well, less-than-charismatic...personality (later on, Hordak & the Horde take on this role). Specifically, he was from the planet Infinita (which the MOTUC has turned into a dimension that Trap Jaw is now from and Scareglow was banished to). That is, until he plopped himself on Eternia. There, he found that Castle Grayskull housed all the power in the universe that he could want & decided he would take that for himself (which would make conquering the universe a snap...literally!). The Goddess intervened by asking He-Man (then just a barbarian, jungle man who's people fought against Beast Man's people on occasion) to protect the Sword of Power--the key to opening Grayskull that had been split into two halves in case something like this ever happened. She gave him the chest harness (which amplified his strength, helped his agility, and sometimes defied gravity--depending on the story), battle axe, and shield. Skeletor did not like this opposition, and instead of just giving up & moving onto the next planet, he decided that the power of Grayskull would be his & he was going to stay until he made it so. So he set up Snake Mountain, told all Eternian villains they were under his command (if they defied him, he would remind them why he was their leader), and continued to wage war against He-Man and the rest of the Masters.
Much as I loved the bungler Skeletor in the cartoons as a kid, I also like the fact that he was originally a much more ruthless creature with no known origin other than being from Infinita. Makes him seem that much more of a bad guy with inadaquate minions rather than himself being one of the reasons he can't gain what he wants.
With Keldor (now onto 200x), he was an alchemist who also practiced magic (or Magick). Hordak found him, enticed him with more power, gave him magic & told him where to look for more (why else would he have been invading Zalesia where he met Evil-Lyn if not to look for magic artifacts or books?). The Filmation Skeletor was more magical with firing bolts of energy from his fingertips or waving open a dimensional portal, than the Jetlag (NAHM) or 200x who seemed only to have magic in his staff (he could call it to his hand, but didn't cast very many spells without it). Being half-elven (Gar), he has a better ability to learn magic than a human would. He and Randor are also only half-brothers (same father, different mothers), although I've read that they were unaware they were related in this series.
As far as the Star Wars thing goes, Vader and Luke were no more related than I am to a rock until Lucas decided to put that plot twist in after making the first movie (3 years later). Looking at the original story for what came to be "A New Hope" (the only SW book Lucas ever wrote; the rest were written by others from his screen plays), there was no family liniage. No Luke & Leia or Vader being fomerly Anakin. Looking at stories and comics that came between Star Wars and The Empire Strikes Back, there was no more Skywalker family. His uncle and aunt had been it.
For a correlation, this is the same thing that happened in the MOTU. Keldor was Randor's long lost brother that was mentioned in one of the last minicomics, "The Search for Keldor", but it was never stated that Skeletor was Keldor reformed. He just said that them (the Masters) knowing what happened to Keldor could destroy him. Maybe (if the original line had continued) it could have gone this way instead: Keldor had accidentally created Skeletor when messing with chemicals or magic that he shouldn't have, and his creation had killed him (like Frankenstein) or sent him to another dimension (like the Enchantress who spirited away King Miro). If they found Keldor in another dimension (or contacted his spirit), he could tell them how to undo what he had done (Skeletor). Just a fun possibility.