"The two brothers-in-law speak with the old aunt."
Translation:Cei doi cumnați vorbesc cu mătușa bătrână.
I gather that "cei" and its variants are the equivalent of the English "the", however only used when naming the number of whatever it refers to.
Yes. Here ”cei” is the defined article, ”those two guys”. Moreover, the most used form here is ”cei doi cumnați vorbesc cu mătușa cea bătrână”. The ”cea” can be omitted (Duo's solution is grammatically correct) because ”mătușa” is already articled (unart: mătușă), but if you omit ”cei” the meaning would be ”two brothers in law speak ...” (undefined brothers in law). If you want to select from many aunts, you will say ”mătușa aceea bătrână”, (ori ”aia”). Without it sounds a bit odd, but it is grammatically right. There is no way to articulate ”cumnați” here without ”cei”, due to the numeral. Without numeral (if they are two, and we know it, without doubt) ”cumnații vorbesc....” but if you add the numeral, you can't say ”doi cumnații vorbesc”, this is incorrect and it sounds very funny for Romanian ears. Of course, you can force it to ”amândoi cumnații vorbesc...”, which is still correct but it means ”both brothers in law...”