Ketubot 82 - First Day of Rosh Hashana - September 26, 1 Tishrei

A case is brought in which a yabam committed to share his brother's inheritance with his other brother but as opposed to that case in Ketubot 81, he did a kinyan to show he wanted to make sure it was effective. However, they still ruled that it was not effective as a kinyan cannot be effective when the item isn't yours to sell. The property of his brother is not his, even once he married the yavama. Once a yabam marries his brother's wife, she is his wife for all intents and purposes. Why was this necessary to say – for what halacha? Why does he not need to give her a ketuba, but she uses the one she has from his brother (the first marriage)? Shimon ben Shatach instituted that all the husband's property be liened to the ketuba. What was before he instituted this and why was it a necessary takana?