Those phrases are examples of the constrasting effect of 은/는. 은/는 indicates the predicate intrinsically belongs to the subject, and this creates the side effect of setting it off against the others. It has the kind of effect as saying "I am all right" (나는 괜찮아요) first and then "what about my friend?" (친구는요?), in a situation involving two friends. So all your phrases need some context to work. You can't say those things as the first sentence. 요 is just a tagged-on suffix to make it polite, not an essential part - you wouldn't have it between friends. * 언니는(요)? = What about (my/your) sister? You might say it when you were expecting to see a lady and her sister but she came alone, in effect "What happened to your sister?". * 잘하기는(요). -기는(요) is a sort of an idiom. A full sentence might be: 잘하>기는 (뭘 그렇게 잘하겠어요)? = How good can I be, if at all? It is a reaction of modesty you have when you receive a compliment. e.g. A: 말을 참 잘하시네요 (You speak so eloquently). B: 잘하기는요 (Well, (thank you, but) I really don't.). * N에 대해서는(요)? is similar (think of it as N에 대해서는 어떻게 생각하세요 / 뭐라 하시겠어요?". It implies someone talked about something but the listener was interested in N, so they're saying "What about (your thoughts on) N?" (I think in most cases, simple "N은(요)?" is more natural).