My kittens from the same litter don't like each other?

My two kittens are 8 weeks old, and are from the same litter. One is male and the other is female. We got the male on Saturday (June 11th), and then decided he needed some company, and went back to the same place and got the female 24 hours later. The male however already doesn't seem to like the female - he's been growling and hissing at her, and if she gets too close, he swats at her. I'm not sure what's going on there, because there's no way he became territorial after just 24 hours and forgot about his own sister, is there?