In German penal law, does an attorney (Staatsanwalt) have to prosecute own relatives (parents children, siblings, etc.)? If no, by what law (I could not find any that would provide them with that right)? If so, are there some case examples?
Attorneys are normal officials (Beamte), so the laws about officials apply to them. So e.g. § 47 Gesetz über die Beamtinnen und Beamten des Landes Nordrhein-Westfalen (Law about officials of the state North Rhine-Westphalia):
(1) Beamtinnen und Beamte sind von Amtshandlungen zu befreien, die sich gegen sie selbst oder Angehörige richten würden, zu deren Gunsten ihnen wegen familienrechtlicher Beziehungen im Strafverfahren das Zeugnisverweigerungsrecht zusteht.
(1) Officials are to excuse of official actions, that are against them or their relatives, if they have the right to refuse to testify against them in criminal proceedings.
(my bad translation)
These relatives are according to § 52 Strafprozessordnung (Code of criminal procedure): fiancée, spouse (even ex-spouse) and any "person who is or was lineally related or related by marriage, collaterally related to the third degree or related by marriage to the second degree."
So attorneys do not have to prosecute their relatives.