I've just watched the scenes where Momsy was searching for a suitable wife.
Ironically, she decided that this Harleen is "the perfect choice bahu" just based on a picture of her and her parents' praises. She knew Happy for almost four years, lived with her, ate with her and always misunderstood her and couldn't understand who she truly is, but this Harleen is a perfect girl just because her parents say so? Of course, they were not going to ill talk about her when they were arranging her marriage.
This marriage is cursed from the very begging as Momsy was arranging it right at the moment that Sandhya was crying for her child. And nothing good can come out when its foundation is set in such a sad moment.