Is what happened with the girl, short of adultery, considered an injustice to her father, and what should I do to atone for this mistake?

Being alone with a non-mahram woman, looking at what is unlawful of her, touching her body, and deriving pleasure from her in any way are all forbidden. Adultery is a graver sin. The act you committed is disobedience to Allah and a betrayal of the girl's father, who entrusted you with his family.