I thought the same too. Didn't she say something along the lines that they are okay with a Punar Vivah now for their son but how come they didn't think the same for their daughter. If she is his Bua how would it be the family's fault that she never got remarried? Wouldn't it be her parents responsibility (Yash's grandparents)? This is another reason why I think that she is Yash's older sister and not Bua as then the blame would be on Yash's parents.