This greatly depends on context, but when I am clearly being introduced to someone's mother, in English, I will sometimes say "Oh. Is this your sister?." as a compliment. The intended compliment being that she looks one generation younger than she clearly is not.
What about in Japanese. Were I to be being introduced to someone who is the mother of someone, and I were to say 「あ、お姉さんですか」 is there any chance that would come off as creepy, impolite, or too awkward? I just want to sound clever and clearly be giving a complement to the older woman and get everyone to smile. Of course, this is so minor I've no problem with never doing it.