What you're forgetting is that Magic makes real life genetic science go sit at the kids' table, while the grownups talk.
Immortal Elves are the children of a pairing between a Dragon and an Elf. I believe Earthdawn, back when it was under the same company as Shadowrun, stated that the immortality remained to the second or third generation from the dragon parent. In other words, Ehran is the kid of a dragon, and Frosty, as the second generation from the dragon, would be immortal, too. But Frosty's kids, if she has any, may or may not be immortal, depending.
And for various reasons that neither Dragons or Immortal Elves like to talk about, Dragon-Elf mating is a big no-no.
Now, what does that have to do with Frosty's mom? Absolutely nothing. As far as we know, Frosty's mom is not an IE, but AFAIK she wouldn't be mentioned anywhere except, perhaps, the original Harlequin module.