If he said that he betrothed her daughter but she said that he betrothed her , he is prohibited to her daughter's relatives and her daughter is permitted to his; he is permitted to her relatives but she is prohibited to his.

(Gemara - Rav Huna): We convince him to give her a get (but we cannot force him as he will thereby become prohibited to her relatives) and if he did give it, we force him pay her Kesubah money.

Question: If he knew that he would need to give her a get and Kesubah money so instead he married her sister, what would be the law?

Answer: We stop the marriage with her sister (which was a certain marriage) because of the doubtful original marriage.