Character Analysis
We're grouping Alice's little brother and sister into one character because really, that seems to be how Alice sees them, too. There isn't much to differentiate between Alex and Tim, other than their gender and… well… nope, that's pretty much it.
Primarily, Alice sees her siblings as perfect little angels who compete for the meager attention they get from their parents, and their perfection only serves to highlight her blunders. She writes:
I know she is always comparing me with Tim and Alexandria and I just simply can't measure up. It seems like every family has to have one goon, guess who's it on this homestead? It's natural to have a little sibling rivalry, but ours is getting way out of control. I really do love Tim and Alex, but they've got plenty of faults too, and I find it difficult to decide whether I love them more than I hate them or whether I hate them more than I love them. (29.1)
When they move to their new town, Alex and Tim have no problem finding friends, but Alice struggles, and wonders why she's the only one. Again, then, their goodness—in this case at adjusting—serves as a contrast to her struggles:
Well, like oil and water, I can't quite adapt or fit. Every so often I even seem to be on the outside just looking in on my own family. How can I possibly be such a dud when I come from this gregarious, friendly, elastic background? (24.1)
Way to be happy for your little brother and sister, Alice… oh wait. Nevermind. Tim and Alex seem like normal, cute little kids. Plus, we're pretty sure they never set out to make Alice feel like an outcast. In fact, they probably have no idea this is how Alice thinks of them, and the poor little buggers are probably blindsided by Alice's escapades.
There is also the potential that Tim and Alex are just one more box on the author's drug-addiction checklist. Sibling rivalry (or at least the perceived rivalry) and feeling like an outcast in one's own family could very well be just another motive the author wanted to throw into the story for good measure.