Do all only children want siblings that badly, or just Kryptonian ones?

More than likely this plot was used more than a few times in Super-books: a ship arrives on earth, with a passenger with super-strength and such; obviously they must be part of the family! From 1969, Adventure Comics #385, "Supergirl's Big Sister!" Written by Robert Kanigher, art by Kurt Schaffenberger.

This one's a bit different than Mon-El's appearance in Superboy #89, as instead of making an assumption based on evidence at hand; Supergirl's alleged sister Kranna claims to be such from the start. (She also tells Kara all this like she could see the panels!) Kranna says thanks to radiation from their parents' lab, she was basically already super by the time she was five, when she was abducted by pirates who erased the memory of her with an amnesia-beam. Feels like there's already a few holes in her story: wouldn't her parents have noticed her room or her stuff? Baby pictures and such? The pirate king intended for Kranna to marry his son when they both grew up, and kept her prisoner with a Kryptonite field surrounding the castle--had Krypton blown up at that point? How would they have known that would contain her? Doesn't matter: Kranna grew up, and developed an immunity to Kryptonite. She had been keeping tabs on her younger sister's career on earth, and stole a rocket and escaped to join her. 

Kranna had also brought something from the pirate planet: coal! Not just coal, but super-coal, which Kranna suggests would be too tough for Supergirl to turn into diamonds like earth coal. Supergirl proves her wrong, crushing the coal into diamonds easily, but is then called away by Superman's super-ventriloquism. Kara flies off, but Kranna wants to put the diamonds away for safekeeping first: yeah she does, since this was an elaborate scam! "Kranna" was really Carol Benton, daughter of an earth scientist; and all her super-feats were hidden devices or lightweight props. Still, Kara wasn't fooled: she had faked Superman's call, since she guessed "Kranna" would laugh first chance she got. She burns the diamonds, and tells Carol where she messed up: the obvious one, she wouldn't have needed a rocket, she would've flown.

Also this issue: "The Jilting of Supergirl!" (Written by Robert Kanigher, pencils by Win Mortimer, inks by Jack Abel.) 'Jilting' isn't used much anymore, is it? As Linda Lee, Supergirl visits the Daily Planet: she had intended to ask Clark, to ask Superman to help with a college building fund, but Superman was actually there. Lois had been having a "girls' talk" with visiting Princess Vee-Ra of Galaxy XL-9, who then flies off with Superman for a night on the universe. Lois then breaks down in front of Linda: she was worried Supes would fall for a woman with powers, since he could never marry Lois without the risk of criminals trying to get at him through her. That has to be remarkably awkward, but Linda tries to comfort Lois, while secretly being glad she would never have that problem. Yes, never, ever, ever...I haven't even turned the page yet, and I'm pretty sure that's going to bite her in the ass.

Linda saves an underground borer from an underground civilization, just like saving a random falling jumbo jet or something, not a big deal; then goes to a dance, where her date is not doing it for her. Likewise, a later date with a star athlete, who can't keep up with a Supergirl. (Not like that; on a bike ride!)  But, new transfer student Steve might be a prospect: good-looking, insanely smart. The guy recognizes Macbeth in Swahili! Of course, he's a star athlete, and when he takes Linda on a rock-climbing date, also has levitation powers that he learned from a guru in India! (Batman must've missed training with that one!) OK, yeah, he's from the underground civilization, the super-powered Prince Raynor of an underground world. Linda is more than sold, and goes with him to the underground city, where everything goes swimmingly--until she gets captured by local malcontents, who want to extort the secret super-power elixir from the royal family. Supergirl is dismayed, she can't break free of the net, and Raynor has to save her, but the engagement is off. Calling her a "hindrance," Raynor cuts ties with her, even though Supergirl is pretty sure that was a Red Kryptonite thing, a one-off. She returns to earth to walk in the rain, in the same boat as Lois Lane; although Raynor seems like a showoff. And the 'malcontents' could've been just anti-monarchy, which I could get behind.