Most children will ask that if they don't have a sibling by the time they're a certain age. I think the way to handle it (provided the parents know this is true) is to say something like, "Maybe one day you will." If the parents know there won't be a sibling - ever - then something like, "Some children have brothers, some have sisters, some have both, and in some families there is only one child." is about the best the parent can do (I think),
Both of those answers are true and reality. I think they're the best kind of answers a parent can give.
Parents should not send the subtle message that there's something "less" about any kind of family mix. Instead, they should send the message that it's nice to have all different kinds of family make-ups, and they're all good. (The child can figure out any disadvantages to any of them once he's older and thinking from a different level of maturity and perspective.)