Futago no Omamori

Mamori, a young girl whose parents have disappeared and whose only blood relative, her brother, is also missing, lives a lonely life. In one of her regular letters from her brother, she is offered a job as a bodyguard for a wealthy pair of twins, so she heads to the interview venue. Although the twins reject her, Mamori passes the interview, but ends up working at the boys' school the twins attend... (Source: Natalie, translated)