Triangle Heart 3 ~Sweet Songs Forever~

A mysterious organization keeps sending intimidating letters to Fiasse Christella, the principal of British Christella Music Academy. The purpose is to find the whereabouts of Fiasse's inheritance.

Although Fiasse already has a bodyguard, Ellis McGaren, she asked her childhood friends, the siblings Takamachi Kyouya and Miyuki to help Ellis. This of course is received by Ellis, as a professional bodyguard, with a cold shoulder.

Kyouya remembered an incident that was happened several years ago, when he was about ten years old. This incident was actually aimed at Fiasse's father, senator Albert Christella. So, does Fiasse's fate and case related to this past incident?

[From AniDB]

The protagonist is the older brother of Nanoha from the Mahou Shoujo Lyrical Nanoha franchise.