From "The Walking Dead", season 05 / episode 09:
Still got a mom and a couple of twin brothers.
I know that it means that in total there are 3 people: the author of the sentence and his two brothers, wo are twins.
But can it also mean that he has more twin brothers, e.g. 4 or 6?
(Mind that this is American English. )
Twin brothers refers to two people. A couple of twin brothers also refers to two brothers; the word twin functions as an adjective modifying brothers and has no effect on the number of people in the group.
If there were five people in the group, you might say:
A mom and a couple of sets of twin brothers.
Or, if only two of the five were twins:
A mom and four brothers, two of which are twins.
Technically, maybe, but it would almost never be meant or taken that way. "A couple" generally means two, but it can be used to mean "a few", especially when the exact number is low, but not exactly known.
In this case, "twin" is an adjective modifying "brothers", so the "couple" refers to brothers. (If someone meant they had two sets of twin brothers, they would probably say "a couple of sets of twin brothers".)
Even without the "twin" (which fixes the number at two), most listeners would expect someone referring to his "couple of" brothers to mean exactly two, because most people know exactly how many brothers they have, and they would presumably not have any reason to hedge. (On the other hand, for instance, someone might say that they had been arrested a "couple of times" to make it sound less than the three or four it actually is. Or that they plan on taking a "couple of days off" next summer, even if they haven't specifically made plans for exactly two days, and it could end up actually being one or three.)
It's a little ambiguous, yes. It could mean...
1: The speaker is one of triplets, and therefore has two siblings, who are boys.
2: The speaker was a single birth, and has two brothers who were born at the same time as each other.
3: The speaker was a single birth, and has four brothers, born in two sets of two.
However, #3 is the most awkward meaning to wedge into the words, though the "couple of twin brothers" almost allows it. The more natural thing to say would be, "...and a couple sets of twin brothers" or "...and a couple pairs of twin brothers." It would be more likely that 1 or 2 would be the case.
“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.
