Me, Myself & I

What is your protagonist's self-opinion?

Does she believe she can never do anything right?  Is she always late?  Does she hate her own body image?  Does she feel eclipsed by her younger, beautiful sister?

Or perhaps she thinks very highly of herself, values her opinions above others, believes in speaking frankly even if it hurts someone's feelings, and feels complacent, sure of herself, and entitled.

We have two very different women here, don't we?

Example A (let's call her Amelia) is going to behave in certain ways that reinforce her self-concept.  She may make remarks such as, "Don't depend on me to be there on time.  You know I'm always late."  Or she may grow flustered whenever given a responsibility.  She will reflect her dislike of her body image by the clothing or colors she wears.  And even if friends invite her along, she may refuse to join in the fun if her sister is part of the group.

Example B (let's call her Beatrice) will always be dispensing advice, whether the recipient has asked for it or not.  She will speak loudly and in a forthright, blunt manner.  She will probably trample over the feelings of people like Amelia, and then become impatient or embarrassed by the reaction she gets.  She will be calm and self-assured in her demeanor.  She will always take the best chair or help herself first to the plate of cookies.  Even if she is overweight, she will not be embarrassed by it, but may instead drop comments such as, "You should know, dear, that men always prefer full-figured women to beanpoles."

In character design, choose that individual's self-opinion and then devise personality traits and behaviors to correspond.  You'll end up with a believable, plausible character.