"Liaj fratinoj estas naŭ-jaraĝaj ĝemeloj."
Translation:His sisters are nine-year-old twins.
It could be, but I think in Esperanto the convention is that only family members and boyfriends/girlfriends have gender-specific words. In all other cases the words are implied to be gender-less. One would probably avoid saying "ĝemelinoj" unless the fact that the twins were female needed to be made explicit.
That's the theory, at least :) In practice, some people seem to expect words without -in- to refer to males; at least, I've had a couple of second-takes when I've referred to a female teacher or doctor as instruisto or kuracisto and then later used ŝi to refer to her and they had clearly been expecting that word to refer to a male professional.
But I think it's a good idea to keep the -in-less words (that don't refer to family members etc.) gender-neutral.