* Πεθερός (petherós): Father-in-law
* Πεθερά (pethera): Mother-in-law
* Κουνιάδος (kounjádos): Brother-in-law (husband's brother)
* Κουνιάδα (kounjáda): Sister-in-law (husband's sister)
* Συγγενείς του/της συζύγου (syggéneis tou/tis syzygou): Relatives of the spouse (this is the most general term and would cover all in-laws)
To use the last option, you'd need to say something like: "Οι συγγενείς της συζύγου μου" (Oi syggéneis tis syzygou mou) which means "My spouse's relatives". You'd replace "της συζύγου" (tis syzygou - of the wife) with "του συζύγου" (tou syzygou - of the husband) if you are a woman talking about her husband's family.