Translation:Τα ξαδέρφια μου
On the one hand, there's the "obvious" answer that if there's one male and one female cousin, for example, then it's "τα ξαδέρφια". However, oddly enough, if someone has two male cousins, it's way more common to refer to them as "τα ξαδέρφια" (neuter plural) rather than "οι ξάδερφοι" (masculine plural). On the other hand, female cousins are more commonly referred to to as "οι ξαδέρφες" (female plural) rather than "τα ξαδέρφια" (neuter plural). It's a bit weird, really.
Does the accent position change, from the single male "ο ξάδερφος" "τα ξαδέρφια"?
Yes, because the accent can't be further back than the third syllable from the end.
So ο ξά-δερ-φος but τα ξα-δέρ-φι-α; since the latter word has one syllable more, the accent can't stay where it was.
Ο ξάδερφος=male cousin, η ξαδέρφη=female cousin, το ξαδέρφι=cousin (unspecified gender- hardly used in singular because you usually specify the gender and you use one of the other two). Those forms form plural as Οι ξάδερφοι/ξαδέρφοι=male cousins, οι ξαδέρφες=female cousins, τα ξαδέρφια=cousins (unspecified gender-used when you are talking about your cousins in general male and female)