Yes, this is entirely possible. A queen is in heat (known as estrus) for approximately 1 - 4 days, during this period she will mate several times, and potentially with several different males.
Cats are what is known as 'induced-ovulators', which means that the act of copulation induces ovulation. Unlike humans, cats typically release several eggs per ovulation and these eggs are released between after she has mated. All of the female cat's eggs are released at the same time and survive for 24 hours. Therefore if she mates with several males, each egg has the potential to be fertilised by any of their sperm. This is known as 'superfecundation'.The number of kittens delivered is generally the same as the number of eggs the female cat releases (although in some cases an unborn kitten can be re-absorbed). In theory, a female cat can have a litter of kittens each of whom has a different father.
Is this why kittens in a litter look so different from one another?
It could be, yes. However, while it is possible for kittens to have different fathers, kittens in the litter can look completely different to one another, but that doesn't necessarily mean they have different fathers. Genes can be dominant or recessive. So, for example, the gene for blue eyes is recessive, meaning a male and female cat who both have blue eyes can only produce a blue-eyed kitten. However, a cat with green or brown eyes (both of which are dominant to blue) can also carry the recessive gene for blue. So female and/or male green or brown eyed cat can produce a blue-eyed kitten. This means the cat(s) with green eyes carry the recessive gene for blue eyes. This is the same for longhaired cats, which is also recessive. One or both parents can be short haired, but produce longhaired kittens if they carry the recessive gene for long hair.
Can brother and sister cats mate?
Yes, this is absolutely possible for feline siblings to mate, as is father and daughter, mother and son. Cats don't discriminate when it comes to who they mate with. When a female is in the season, she will mate with any available male (well most, some cats can be pickier). This is why it is so important to get all cats in your household desexed before they reach sexual maturity at around six months of age.
What happens if a female doesn't get pregnant?
If the female doesn't get pregnant during a heat cycle (usually because she has no access to a male), she will come back into heat a 1 - 2 weeks later. This cycle will repeat until she either falls pregnant or the season's change. Cats are seasonal maters, and this cycle is tied to the lengthening hours of daylight. So they come into heat in spring and autumn, once the days shorten, her cycles will usually stop, although some cats will continue to come into heat.
Image courtesy of Michelle.