Well, unless they are monozygotic twins, then the sex of the second is determined by the sex of the first and the probability of the second being a girl is, for all practical purposes, 100%
True, but we were to perform the math based entirely on the information at hand. The odds of identical twins, one kid being horribly mutated so as to fit as a "both" or a "neither", etc etc. are so incredibly small it's unlikely to affect a percentage
when displayed as ("###"), which is how we've been displaying it.