Honestly, there is nothing to refute your idea of adventure, other than to point out that it is entirely subjective. What we can infer about Katie is hobbled by how little we get to know her. She could be "a total, boring, straightedge" as you call her but, again, she seems to be perfectly content to be so. Perhaps, knowing the way her parents react to things, she simply chooses to filter what information they had access to. The contrast with her younger sister (who, it should be noted, wasn't really doing anything crazy and adventurous until she had the good fortune to meet Lonnie) strikes me as another unfortunate instance where Katie exists merely to shed more light on Sam. We never even find a letter directly from Katie to Sam. Their relationship, one that is impressed upon us as being particularly close, is the one, glaring example where the game chooses to tell instead of show. Which is fine as this is, on the face of it, Sam's story. It's just awkwardly lopsided, especially considering how it's the top-layer narrative, leading to a lot of less than satisfying conjecture.
I know this is a game/story where it's really easy to read too much into the pieces that are given, but I suspect that Sam was always the more adventurous sibling, even before Lonnie. My key piece of evidence is her incredible short story when answering the sex ed question. The kind of 9th grader who writes that is a natural troublemaker of the best variety. The second piece of evidence is that it's her who chases after Lonnie initially. She identifies Lonnie as someone she wants to know, observes her habits, develops a plan to get to know her, and then executes that plan. The parents blame Lonnie for Sam's troubles that year, but Sam was already on a path to become a troublemaker. They even used the classic parent line, "You never had these problems before her", which is often just parents trying to outsource a problem rather than take a sober and honest look at who their kid is. Lonnie and her fueled each other's mischievousness, rather than it being a case of one influencing the other more.
Again, adventurousness being subjective. I think at this point we may be arguing our own internalization of the story, in which case we are doomed to chase each other in circles with no end game in sight!