Maseches Yevamos, Chapter 4, Mishnah 4. The Mishnah addresses the status of the yevamah after the yavam has fulfilled the mitzvah of yibum with her, and it rules: "Kenasah - harei hi ke'ishto lechol davar" - once he has married her, she is like his wife in every respect. From the moment she enters his household, she is no different from any other woman he might have married, in every matter.
"Harei hi ke'ishto lechol davar" - she is like his wife in every respect:
Divorce: If he wishes to divorce her, he divorces her with a get alone, and no additional chalitzah is required - the mitzvah of chalitzah is finished, and she is his wife in every respect.
Remarriage: He is permitted to take her back and marry her again, just like any woman he has divorced.
We do not say that some trace of the prohibition of a brother's wife remains upon her, as though she were fundamentally forbidden to him and was permitted only temporarily for the sake of the mitzvah of yibum. That is not the case: she is like his wife in every respect, and there is no difference between her and any other woman he married.
"Uvilvad shetehei ketubatah al nichsei ba'alah harishon" - provided that her ketubah is collected from the property of her first husband:
Here the Mishnah adds one qualification: her ketubah is collected from the property of her first husband. The basis for this is that the ketubah rests upon the inheritance that the yavam received from his deceased brother, and it is that property which is liened to her ketubah, and not his own property.
However, if his brother had no property at all and left nothing behind, the Sages said that he must write her a ketubah from his own property, since a woman does not remain married without a ketubah.
In summary: In this Mishnah we learned that after yibum the yevamah has the status of his wife in every respect - divorce with a get alone without chalitzah, and the possibility of taking her back - and we do not view her as one who was permitted only temporarily. The only qualification concerns the collection of the ketubah, which rests upon the property of the first husband that the yavam inherited; and where there is no such property, the yavam is obligated to write her a ketubah from his own property.