Need some help with my main character?

Lots of feelings! Anger, for one, frustration, for another. Frustration not just at the situation, but at your main character's inability to escape it. Stifled might be another, since your character should have her own goals, dreams, ambitions, and talents that are not being pursued as much as they could be. You could also make her extra driven to succeed - maybe she's working extremely hard to achieve success (school, sports, magic, dragon riding, whatever) to try to get attention. I'm assuming she is torn between wanting to help and wanting to be free of the situation, so there's that as well. It doesn't sound like a purely abusive situation, so there must also things that keep her there - love, sympathy, pity, compassion. Good times that they have and have had together? Positive family bonds? Any family situation is complicated, and this sounds more so, so there's probably a whole jumble of complexity to work through.

As far as fears - well, big one is right in front of her: the fear of death. Her sister, who's about her age I assume, is dying. Again, assuming she's young, she's facing her own mortality, which is hard at any age, but maybe particularly at a young age. Then there's fear of suffering, if her sister is suffering that's another awful part of this situation. How about the feeling of helplessness, which could be a fear for later in life because she now knows that things happen you can't control? Fear of letting her family down when they need her? Fear of her true self - afraid her true self is selfish and weak and maybe evil for having these thoughts? I personally think the situation you describe would make it really appropriate for these deeper fears, even more than "not being good enough". Because the object of her envy/competition is a dying sister, I think it's a little different than if it were a sibling who was seen as "better" for something they could do, how they looked, etc. I'm not saying there wouldn't be that feeling of frustration/disappointment for not being able to get her mother's attention. But watching your sister die, and the terrible desperation of their mother, I think that is an experience that goes beyond just sibling rivalry. If she does have that as a fear, maybe it's a surface fear and underneath is a deeper one brought on by this terrible situation. But again, family is complicated!! And every dynamic is different, every individual reacts in their own way, and I don't know the full situation of your story.

 

It's all up to you! These are just some suggestions. Your character is your creation, and a unique person, so get in her head and go through her day and imagine what she would do

 

I would include love - it complicates the MC and makes the reader a little more sympathetic towards her. I wouldn't like a character who resented and envied her dying sister.

 

How about protectiveness and sacrifice for their sister. Guilt for not being able to do more for her sister or mother. Also is the younger sibling really being favoured or is that the MC's perception. The perception will change as she grows and understands that sick people, especially children, need more attention. Perhaps the mother thinks the MC is highly independent and able to do everything for herself. I'm simply taking a look from a possible other side. Perhaps you could find an article or biography of someone who lived with a similar situation.

 

Show, do not tell. Whatever you do, DO NOT start telling the reader "she is out to prove herself but secretly insecure" or something... give her situations and see how she reacts to them. Soon, we will know who she is. REMEMBER, you can write out little paragraphs of a story you DO NOT mean to put in the story, and you will find out who she is from how she reacts. Eg, a friend's bf is cheating, someone wins a prize she thinks she should have won, she is served food she hates by a host, etc etc...

Why not give her a personality test? takes 12 minutes, and let that also feed into your idea? Free personality test | 16Personalities

 

Maybe the character is motivated by a since of guilt. Maybe he has lived most of his life trying to get out of the shadow of his sister, to prove he is just as capable to his parents only to find his sister ill and getting worse. He may blame himself for allowing his obsession of self-improvement to prevent him from growing close to his sister and now finds himself with very little time to bond.