and Gil gave this answer: MF FM FF and MM are the possiblities, we'll go with boys, so FF is out, and MM is once of the 3 options, so 1/3 chance. this is a well-known answer.
anyway, that's WRONG
it *is* 50%. the above answer does not take into account anything about the possibility of meeting either sibling. the MM option is really *two* options, meeting *either* the younger or older sibling.
another way to put it, is: if you've met the older sibling, the options are FM and MM which is 50/50 either way. and if you met the younger sibling, the options are MF and MM which is also 5/50 either way.