I share the room with my sister.
共同 sounds unnatural a bit although it’s understandable. 共同 is formal-sounding, so I don’t use 共同 when I describe my sister and myself.
私は妹 or 姉と同じ部屋だ。（わたしはいもうと/あねとおなじへやだ）= (lit.) I am in the same room as my sister.
sounds more natural in Japanese. 同じ部屋だ not only means “to be in the same room” but also “to use the same room for a certain period”. It also suggests that someone (i.e. parents) assigned the same room to my sister and me.
私も妹も自室がない。（わたしもいもうともじしつがない）= (lit.) Neither I nor my sister has a personal room in our house.
This is another translation. It doesn’t directly mean that I share the same room with my sister. I may share a room with my brother while my little sister is still sleeping with our parents. But 自室 is a frequent expression you guys may want to memorize for your future use.