Shulchan Arukh, Even HaEzer 162/שולחן ערוך·אבן העזר קס״ב

2When a man performs chalitza (ceremony to undo the levirate bond) on a woman bound to him for levirate marriage, that woman and her co-wives become forbidden to him and to all his brothers by positive commandment.

3A man granting chalitza (separation ceremony for a woman bound for levirate marriage) to his [late] brother's wife, is Rabbinically forbidden to marry her relatives, even second degree relatives. Similarly, she is forbidden to his relatives. But his brothers are permitted to her relatives. Note: Even if he performed chalitza out of doubt, he is still forbidden to all her relatives (Mordechai, beginning of chapter haCholetz).

4A man is permitted to marry the sister or other relatives of the co-wife (tzara) to his [late] brother's wife to whom he had given chalitza.

5If is forbidden to marry the co-wife of any of the relatives to his [late] brother's wife to whom he had given chalitza.