[His expression twitches slightly at the word 'twin,' though exactly what emotion is trying to break through there is hard to tell.]

Siblings are any individuals sharing the same parents - it gets somewhat complicated by half-siblings, adoptions, and something similar to the sense you're using it in. Twins are any pair of siblings born at the same time, identical or not - we also have triplets, quadruplets, and so forth.

If your social system isn't blood-relation based, what is your primary social framework?