The story of Bidaai could progress like this. There are two possible ways, pls feel free to comment.
1. Sadi will inform at home that Vasu wants her to marry her mentally unstable son and taking advantage of this situation, nani will trade saying if Sadhna has to marry Aleek, then, Ranvir should marry Ragini. If this happens then, the story will progress as predicted and after all the struggles for Sadhna, Aleek will finally get well and they will live happily ever after. In between, there might be other problems also, like ppl pointing fingers at Sadhna as Ranvir seems to have taken a liking for her from the day he set eyes on her. This could also lead to a temporary misunderstanding between the two sisters as Ragini could feel that Sadhna is unhappy in her marriage and hence trying to get close to Ranvir. But hopefully, this misunderstanding is soon settled and Aleek gets well too.
Another way the story could progress is - Sadhna and Ranvir get married and somehow (not sure how) Ragini and Aleek get married. Then, in the Rajvansh house, everybody is giving royal treatment to Ragini bcos she is the one who has married Aleek and again even in her new house Sadhna is given step-motherly treatment. Vasu will always hold a grudge against her for not marrying Aleek and whatever the case may be , Ragini will always be given more attention. In this way, Sadhna will again struggle. But, eventually Vasu will start liking Sadhna as well and alls well later on.
If Ragini marries Aleek, then, Aleek will get well faster and if Sadhna marries Aleek, he will get well very slowly. Being the main protagonist, Sadhna has to suffer the most. Wat say folks ????