Dunkel can refer to hair, skin, sky - it simply translates to "dark" . "My sister is dark" has the same meaning as Meine Schwester is dunkel. If it is used in that way it maybe means something like her soul or demeanor is dark or she is attracted to the dark (like gothic or something), but that sentence in german (Meine Schwester ist dunkel) has no clear meaning without the context it is mentioned in. if you found it somewhere in literature you might have to look up the context to guess the meaning. If you wanted to say she has dark skin, you`d say Meine Schwester ist dunkelhäutig (if dark skin means she might has a natural darker or black skintone) , if you want to express she is suntanned you`d say Meine Schwester ist gebräunt / Meine Schwester ist braun geworden . If you wanted to say she has dark hair you`d say Meine Schwester hat dunkle Haare. Hope that helped, regards, Bella

By that context it just means that the sister is the complete opposite to A. Speaker B says A is tall (groß) , blonde, skinny (schlank) and the said sister is small (klein), dark (dunkel) and chubby not-so-skinny. So in this context I would say it means the sisters hair is darker colored like black, brown, something like that, because where B. says dark in the description of the sister, B says blonde for A which is a haircolor. Hope that helped, regards, Bella