When two high school enemies discover their single parent’s newfound love, they must team up to destroy it or end up as step brothers

Great set up. I can see the movie already.
Suggestion:
"Two high school rivals"
"two high school arch-enemies"
Plural form of nemesis = nemeses. So nemesis is not, technically, accurate.

"When their parents begin dating, a high school loser and the head cheerleader who torments him must team together to break up their parent's budding relationship or end up brother and sister."