Sage

First off, thank you anon! :3 You’re lovely, and I’m glad to help.

Heather was introduced long before modern cosmic Marvel really got going, in Iron Man 54. She had some skeevy storylines, though including one where she mind-controlled Thor into loving her. It’s unclear, though, how much of this was really her, and how much of it was due to evil cosmic possession. (Yes.)

Her redemption arc really begins in The New Defenders 125, ending in her apparent death in 152. In Solo Avengers 16, it’s revealed that her soul survived, and in 20, she and her cousin - Pamela, AKA Sundragon - adventured together in space. 

Heather’s next appearance is in Infinity Watch, as the keeper of the Mind Gem. Next was Captain Marvel (1999), in which she attempted to aid Genis-Vell and fell for Rick Jones’ wife, Marlo. 

This series is also Phyla’s first appearance, in issue 16. She’s Genis’ bio-engineered sister, determined to take the title of Captain Marvel because poor Genis is a hot mess. She and Heather start dating in issue 25.

Issues 1-25 are collected in four volumes:

Nothing To Lose
Coven
Crazy Like a Fox
Odyssey

Finally, we’ve got the beginning of modern cosmic, with Annhilation. Phyla and Heather don’t come into it much until the end; they play a bigger role in Annhilation: Conquest, GotG, and later events. I recommend reading everything, though, because it’s all excellent.

Here is a great reading list. o/

Unfortunately, neither of them are featuring in current comics; Phyla is dead, and Heather is…somewhere.