To dream that a dog bites your on the leg, suggests that you have lost your ability to balance aspects of your life. You may be hesitant in approaching a new situation or have no desire to move forward with your goals. Alternatively, it symbolizes disloyalty.
Dream is the best thing in every one life, but some times, it will make you more fear and living a dare life. This dream is just arise in your sister sleeping time, due to some of the fearness about the dogs or may be she is suffering with fearness, when some one suffering from dog bitness. So don't worry about this dream. Just cool and keep a happy life.