First of all, this book is really poorly titled; a more accurate version would be Friends with Your Siblings, or, even more accurate but completely unwieldy Friends with Your Siblings and Also New People Whose Only Friends are Their Siblings. (I can see why they didn't go with that one.) All the strengths (and weaknesses) of the book stem from its use of semi-autobiographical detail, and pitching it so broadly with such a generic title just sets up expectations that won't be met for a lot of readers.
Unlike many of those readers, who don't believe that such a thing is plausible, I was also homeschooled until I entered public high school! This is actually pretty common among homeschoolers who do it for reasons other than to hermetically seal their children from a sinful world; high school is when having specialist teachers for upper-level subjects outweighs the benefits of individual attention and personalized curriculum for your average homeschooled kid. I really liked the bits about Maggie being at a new school and new to school, especially her hand-drawn and annotated maps of the school building. It's always great when people say your actual life experiences are unrealistic! What they probably mean, and what I don't entirely disagree with, is that the author doesn't always sell you on those details in a believable way.
Contre the title, Maggie only makes friends with one boy at her new school, and it's a boy who has unresolved history with Maggie's oldest brother and whose only other friend is his own little sister. The importance of your siblings as your primary social circle is at the core of the book, and it's very sweet.