“Twin” is a noun, sometimes used in adjectival role, indicating one of strictly two (that is literally the meaning of the word: one of two), see Oxford, Webster, Wiktionary or your preferred dictionary. As such, to exist a single twin requires another matching twin, hence there cannot be less than two. Per definition there also cannot be a single set of more than two twins each of which matches all the rest.
Now, from context we have our object which is “a couple”. One of the possible meanings is “indefinite small number”, which is usually indicated synonymous to “few”. The latter, however, does not provide definite limits.
Anyway, our object is “a couple” and it bears further identification “of twin brothers”, which is the object complement. The closest structure I can imagine to describe this is when “of brothers” is the object complement and “twin” assumes some sort of predicative role to the object complement.
In the end this leaves us with the following semantic structure:
“Has an undefined small number of brothers who are twins.” To qualify as “twins” they have to meet two conditions:
1) Come strictly in pairs;
2) No more than two individuals matching each other.
Having no defined upper limit to their count (definition of “couple”), they can be two, four, six, or any other positive real number (cannot have a negative, complex or rational number of people) divisible by two, which number can be identified as “small” in your particular case.