Character help?

[dante]

I'm planning out characters for a co-write between me and my friend. I'm looking for some ideas to add to them. It's like a modern version of the biblical story of Cain and Abel but with opposing powers.

Alumina Sage Russ: Aka: Mina (Guardian of Light)
She has medium, curly chestnut brown hair with beautiful, amber brown eyes and a light tan skin tone. She is a social butterfly. She loves attention and is always with her friends. She is a popular girl that a lot of people enjoy to be around and she is a bit of a loud mouth. She will speak her mind and has a bit of attitude. She looks more like their father, with a shorter, stockier build to her. She has a cute face compared to her sister.

Kerrigan Mae Russ: Aka: Kerri/ Kerri- Mae (Harbringer of Darkness)
She has long, straight red hair with deep forest green eyes and pale skin tone. She is a loner, always hiding from everyone including her family. She doesn't speak much to anyone, often alone by herself. She is a beautiful girl but hides behind baggy clothes. She is taller and slender, resembling their mother.

Questions:
- What fear/ rare disorder can I give Alumina that will impact both of them: either hindering or helping them?

- What dirty little secrets can I give them both?

[Brenda]

I don't really see how this is like the story of Cain and Abel. That story is entirely about core pride issues (Cain having an abundance, and Abel blessed to be free of it). Are you sure you don't mean Jacob and Esau, who are actually twins and both cope with jealousy, bitterness, selfishness, treachery, and greed? Because that would be a lot easier to model your story after, if you want both your characters to have a lot of dark facets. In that case, Alumina would be your Jacob and Kerrigan would be your Esau. (I would encourage you to use the full names you've chosen, as the nicknames seem to take all the dignity out of them.) Esau was dad's favorite, but Jacob was mom's. Esau was outdoorsy and athletic, but Jacob was smaller and crafty. Esau's weakness was most likely addiction, as he was all about the instant gratification--selling his birthright for a bowl of stew. Jacob's weakness was pride, as he knew he was clever and used it to get a leg over everyone he knew. Jacob was intended for great things by a great many people, but his reliance on his own trickster capabilities fouled that up repeatedly. Esau was shut out of the blessings Jacob received. He coped, and made a life for himself, and even found a way to make way for forgiveness later in life, but he never embraced good as Jacob finally learned to do in the end.

EDIT: I don't mean to NOT answer your question. It's just that using Cain & Abel as a model gives you very little to work with--especially for where you seem to be taking this.