Shared describes something you have in common with another person. You can describe the people both you and your sister know well as your shared friends.
Things that are shared are mutual — or jointly owned. A shared car is used by more than one person, all of whom can drive it and are responsible for buying gas and paying for insurance. If you sit at a big, shared table in a restaurant, you'll probably be seated next to people you don't know. Shared has an Old English origin, the word scearu, or "division, part into which something is divided," from a Germanic root word.