Ruling on removing pubic hair for an old man who is unable to do this for himself

When my father became very old and was no longer able to take care of his own personal hygiene needs, I used to trim his moustache and shave his pubic hairs for him, but this meant that I saw his ‘awrah (private parts, which should remain covered) without intending to. Is this a sin on my part or not? I heard that the person who sees his parents’ ‘awrah has to fast for two months – is this correct or not?