Pregnancy is a common result for in both dogs and cats.
Dealing with puberty and sexual maturity is a common situation for people who adopt brother/sister littermates or who have a litter of puppies or kittens in the home.
The age of puberty varies -- it can sneak up on you, resulting in pregnancy and more puppies or kittens. Generally speaking, dogs and cats enter puberty around six months of age, but it may be as young as four months of age.
The best prevention for unplanned pet pregnancyis to spay and neuter. Animals become very creative when a female is in estrus. Keeping them separate, as pets living in the same household, may prove difficult to impossible.