Is there sin upon one who informs the family of a minor girl about her relationship with a married man, when he possesses material evidence proving that?

Whoever becomes aware of such a matter concerning a girl must advise and admonish both parties to repent to Allah. If they repent, then it is obligatory to conceal their affair. If they persist, they should be threatened with informing the girl's guardian or escalating the matter to the responsible authorities. It is also permissible to hint to her guardian to monitor her and inform him of what is happening, provided that a greater harm is not likely to result.