Well, that would be much more idiomatic English but there's no equivalent to "your" in the Swahili sentence. It could mean "my sister" or "our sister", for example.
I believe that would be "Wewe na dada yako ni watoto". But I do understand the translation to English sounds strange. In some countries "sister" can be used to describe a younger or similar aged female to yourself. In that type of context it makes sense.