Since Gauri was very little, maybe she knew her as just jiji or didi and not remember her name. And though we see Gauri talking consciously, she is probably thinking in her mind or to herself. Cos her mom said she does not know who the girl in the photo is. If gauri had talked to her mom about it, she would have known it's her sister. As for Shivaay asking her mom if they tried to find her, it's not easy unless you have money and resources which they did not. So maybe now gauri or Shivaay or someone till try to find where she is.
OK, I dint see today's epi yet. Gauri does not know its her sis? 😲