사촌들이 학생들이에요. (O) My cousins are students. or 사촌들이 (모두/다) 학생이에요. (O, I think this expression is better than above.) My cousins are (all) students.

Hi there. 들 is rarely used in Korean, at least in all the cases where we would expect a plural in occidental languages like English. It's used to put emphasis on the fact that the element designated is plural, but it's not a requisite in Korean. The reasoning behind it is that the number of that element can easily be inferred from its context. In my experience, I've only seen 들 be consistently used with expressions like 여러분 (여러분들(은)). Hope that helped.