No, there's no discussion at all about Antigone's and Ismene's appearances in 'Antigone'. Sophocles [496 B.C.E.* - 406 B.C.E.] gives readers absolutely no clues as to the sisters' ages or looks. There isn't even a clue as to their exact ages or birth order. But readers may guess that Antigone is the elder sister because of being engaged to be married and because of the confidence with which she handles their interaction.
*Before the Christian Era
Yes, I sympathize with Ismene's caution. Particularly do I feel sympathy with her if what I suspect is true: she's the younger sister. Also do I feel sympathy with her because of the intolerability of the situation in which she finds herself. Does she respect her sovereign or her gods? Does she respect those who are dead or does she think of the survival of the family?