I understand that these are flaws that she will overcome, adding a satisfactory character arc, but the real question is this: what are her limiting beliefs that could lead to losing her brother? Certain emotions can create thoughts and beliefs, and certain beliefs can cause certain emotional character flaws.
Maybe she is selfish because she believes that no one else matters nor is worth helping out, because it's a dog eat dog world out there. Maybe she's irresponsible because she believes that things will sort themselves out and that she doesn't need to worry and do anything about it.
An unrelated example: a drug addict would believe that life is too mundane, unpleasant or harsh to deal with without the aid of heroin...until they meet the man/woman of their dreams and realize that they don't need it in their life and that they will lose their love if they don't go clean.
IMHO, its best to stick to one of the flaws you mentioned in your logline. Remember the belief system she would have.? A simpler way of viewing it, is that the character believes a lie, and that by the beginning of the 3rd act, she learns the truth.
Anyway this all my personal opinion.
So what is the appeal of this 18year old? She is both selfish AND irresponsible, what are the positive things about her that don't allow her flaws to undercut her appeal?