Character Analysis
As anyone who's ever had a sister knows, it isn't always smooth sailing. In Lou and Treena's case, however, it seems like the forecast calls for choppy waves more often than not.
Sister Act
The root of Lou's issues with her little sister is that Treena is the favorite in the family. She's the smart one. She's the ambitious one. She's the one who actively works to achieve her dreams. Of course, Lou is quick to point out that Treena is also the one who had a kid out of wedlock, using this as evidence that little sis isn't as great as everyone thinks she is.
That being said, Treena is a supportive sister, for the most part. She's the one who gets the idea to take Will on a series of trips to teach him how to appreciate life again. Sure, Lou ends up hating most of Treena's ideas, but it's the thought that counts, right? Lou walks away from this conversation with the feeling that she "was very glad indeed that [Treena] was [her] sister" (9.157).
Sister of the Year
In a similar way, Treena shows her love by supporting Lou's decision to go with Will to Dignitas. That's a pretty different reaction from Mrs. Clark's, who is so upset by the news that she tells Lou she shouldn't return home if she decides to go. That's harsh. It's Treena who backs up her sister big-time: Treena knows how important Will is to Lou, even if she admits that she "had never loved a man like [Lou] seemed to love Will" (25.117).
That's what sisters do. They drive us crazy sometimes; they get us angry sometimes; they make us rant and rave like cast members from Real Housewives sometimes. But they always have our backs when we really need it, and that's what really counts.