A while back I answered a question from a reader who wondered whether kittens from the same litter could have different fathers. The answer was yes. I also stated that human twins could, in the proper circumstances, be half-siblings.
From the May 29, 2009 issue of The Week:
[It was a good week for] ‘Heteropaternal superfecundation,’ after Mia Washington of Texas entered the biology textbooks because a DNA test revealed that her 11-month-old twins have different fathers. She admitted to have been sleeping with two men when they were conceived.