Nothing to See Here by Kevin Wilson Calvert Library's Book Bites for Adults

Lillian and Madison were unlikely, yet inseparable friends at their boarding school. Then Lillian had to leave the school unexpectedly in the wake of a scandal, and they have barely spoken since. Years later, Lillian gets a letter from Madison pleading for her help. Madison's twin stepkids are moving in, and she wants Lillian to be their caretaker. However, there's a catch: the twins spontaneously combust when they get agitated.