I certainly understood each character's point of view. Pru was lonely and could have used someone close to her that she could rely on (someone closer than her friend). Arlo was a complex person and, while he and Sarah did seem to have a bit of a bond, it wasn't a strong one that could be relied on, thick or thin, and mostly complicated Sarah's relationship with her family.
I have one younger sister; we're not close and we never have been. Although we're less than two years apart in age, we always had radically different interests and that's held true as we've aged. She was always very social with lots of friends and was very physically active, while I was a totally introverted bookworm. We didn't have much interaction growing up, either positively or negatively, so our relationship these days is really one of benign neglect.