I’mma tackle both of these asks at once!
Because my focus is a lot more on Helen/Gail’s story and how it connects to the movie timeline (as well as the after-movie timeline with Voyd, etc.) I haven’t dedicated tons of thought to Helen’s family and childhood. I have all ideas she was a middle child in a family with several children, and this contributed to her ambition and desire to the best at everything she set herself to, so she’d stand out.
My headcanon is that Helen and Cass are fraternal (aka non-identical) twins. I think they were pretty close growing up and without a doubt Cass would have been aware of Helen’s powers (probably since Helen herself was!)
Cass doesn’t have powers, but she’s fine with that. She’s not super interested in adrenaline, glory, fighting crime, or having a secret identity. That sounds like such a headache! Cass is more concerned with practical things, like working towards her dream of opening her own little cafe that serves perfect scones and hosts beat poetry nights. She might not be a top student overachiever like Helen, and her dreams might not be as glamorous or sky-high, but she still finds her niche and what makes her happy.
Helen’s parents got divorced when she was young, and in doing so split up the kids. Cass ended up with one parent and Helen with the other, and this accounts for why Helen has a different accent. It also accounts for why Helen is very wary of saying “I love you” back to Gail when they start dating- her parent’s divorce jaded her on commitment and so she’s scared of it. She had a front-row seat to the ugly fallout of her parent’s relationship and the traumatic effect it had on her family.
Helen’s worried that if she does get serious in a relationship, it’ll fall apart, so it’s easier to tell herself relationships are easy come, easy go. She can’t get hurt if she never lets herself be fully invested. Of course, a lot of this is an act that Helen puts up to protect herself. She’s not nearly as unattached as she pretends to be, especially not to Gail. When Gail said, “I love you” it was like taking a sledgehammer to that whole charade and it forced Helen to really self-reflect.
Helen is perfectly comfortable being flirty and provocative, because it’s like playing a game. But to Helen, the “L” word is a major commitment, and something that makes her feel very vulnerable to admit. It takes some time (and some close calls during dangerous hero work) for her to admit the true depth of her feelings and say “I love you” back to Gail. This isn’t because she didn’t love her, but because she wanted to be absolutely 100% sure that she meant it completely– and because she had to overcome her fears that serious relationships inevitably fail. (This makes it extra hard for her when Gail “dies”– Helen feels like her worst fears were realized and she’s being punished for letting herself get so emotionally attached to a partner.)

