Grasshopper Jungle's main character Austin is horny. All. The. Time. And he  makes sure the reader doesn't forget it by mentioning it. All. The. Time. About halfway through the plot moves beyond his libido, a bit, and begins to focus on the mutant grasshoppers taking over the world. It becomes a more engaging story line, although with some big holes. Like, how did the flamingo and movie reels get on the roof??? There are a few redeeming moments. Austin is awkwardly uncertain about his sexuality. He loves his girlfriend. And his best friend Robby. And he is sweetly conflicted and confused about his own identity. And, also,  they both make him horny.

Sister Mine is about formerly conjoined twins Makeda and Abby. They are the daughters of a mortal mother and demi-god father and were separated shortly after their birth. Abby is a talented musician and Makeda is good at nothing. Abby is full of mojo, demi-god superpower, and Makeda has none. They have an odd (to most of us) but typical (for mythological characters) incestuous relationship together and with their cousins, but unlike Grasshopper Jungle, this is a minor detail of their lives that adds to our understanding of them as members of an immortal family, not a distraction that gets in the way of the story. It has a sweet happy ending in which everyone finds their special gifts and their place in the world, although it came too quickly somehow and wasn't very satisfying.  Overall, the book is solidly "okay."

My vote is for Sister Mine, I guess.