The main RuneScape Wiki has an article on: Frank Sinclair
The Old School RuneScape Wiki also has an article on Frank Sinclair.

Frank Sinclair is a non-player character, one of Lord Sinclair's children and a suspect in the Murder Mystery quest at the Sinclair Mansion. He owns a silver pot, which is used to produce his fingerprint in order to compare it with the murderer's fingerprint.

Dialogue

This article on a non-player character has an associated dialogue page.

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