"men sin to his own detriment"
*their
but I don't know what do "sibi" and "ipsis" mean respectively.
Sibi as I understand can only be dative, but ipsis could be dative or ablative, or maybe just emphasizing sibi? I have no idea please help.
Sibi means "to/for themselves", and
ipsis, which agrees with
sibi in the dative, adds emphasis to it — it's more or less like "to/for their very selves".
I guess the most literal translation you can get is "to themselves themselves", but of course that sounds ludicrously repetitive in English. There is no repetition in Latin, though, since, as you can plainly see,
sibi and
ipsis are entirely different words. They correspond to different uses of the one English word "themselves".
English
-self words are used in broadly two ways:
1) As reflexive pronouns, when the subject of a verb is doing something to itself or something that involves itself, as in "I hurt myself", "He looked at himself in the mirror", "They are pleased with themselves". This in Latin is conveyed by the reflexive pronoun
se (dat.
sibi).
2) For emphasis on the identity of a person or thing, meaning "in person" or "that very person or thing", as in "Do it yourself!", "I saw the king himself", "Though I don't like how the book is presented, I enjoy the stories themselves". This in Latin is conveyed by
ipse, ipsa, ipsum.
The two kinds of
-self words are rarely combined in English, because it would cause awkward repetition. It can happen that both ideas are kind of present at the same time, but it's implicit, with only one
-self word being used.
But since they are different words in Latin, Latin has no problem about combining them to convey both ideas at the same time.