This might be totally unhelpful description but here goes:

english film

It involves 2 teenagers (1 boy, 1 girl) who I believe worked at some sort of waterpark/ or theme park (maybe lifeguards??)

They are or become friends and a lot of the movie involves these 2 characters and the girl’s younger sister, whom I believe does kiss the boy at some point.

i would hazard between 2000-2010

sorry for the vagueness but any guesses at a name would be gladly appriciated. Thanks.