A yavama (a widow who must perform a levirate marriage with her late husband's brother) who is forbidden to the yavam (her late husband's brother), whether through a negative commandment [in the Torah], or a positive commandment [in the Torah], or is a secondary [relative of his] on a Rabbinic level, she performs chalitza (action by which a yavama is permitted to marry a non-yavam) and does not perform yibum (a levirate marriage), and her co-wife performs either chalitza or yibum. And her performance of chalitza does not exempt her co-wife [from having to do chalitza or yibum], but the chalitza of her co-wife exempts her. And if he violated [this rule] and engaged in sexual relations with her, he [thereby] acquires her [as his wife] and this exempts her co-wife. And if he wishes to divorce her, he divorces her with a get (a divorce document)
If she was forbidden to her husband by a negative commandment [in the Torah] or a positive [commandment in the Torah] or a Rabbinic prohibition, and permitted to a yavam, she can perform yibum; except [in the case of] a man who brings back his divorced wife once she was married and he dies, she performs chalitzah and not yibum, and her co-wife performs either chalitzah or yibum.
The sisters of a woman that you did halitzah with are forbidden to marry, by rabbinic decree. Therefore, if the sister of the halutza "fell" to the yabam (meaning, she is married to another brother of the yabam and her husband died), they cannot marry and do yibum (since her sister already went through the process). She needs Halitzah to permit her to the world. Her co-wives (other wives of her husband) have the same law, by rabbinic decree.
A woman who has double levirate ties, either she or her co-wife undergo halitzah, but not yibum. How is this (what is an example)? 3 brothers are married to 3 unrelated women. One brother dies and one of the remaining brothers gave the widow "ma'amar" (kiddushin-style money which creates a partial bond but does not effect full levirate marriage), but dies before they have relations. In this case, the woman is now a double yebama to the remaining living brother and he must do halitzah- he can't marry her.
If her gave her a get, and explained that it was to invalidate the ma'amar (money given to partially effect levirate marriage), he thereby abrogated the ma'amar, and she is bound by only a single levirate bound, and she remains permitted for levirate marriage [to another brother]. But if he gave her a get per se [without specifying that it was intended to abrogate the ma'amar] then she is forbidden to undergo levirate marriage.
A minor who had relations with his yevama, and then died, she must undergo chalitza and may not undergo levirate marriage. For his intercourse does not effect a full acquisition [of the yevama], and she now has dual levirate bonds (via the death of her original husband, and the subsequent death or his minor brother who was also her partial husband).
She who is half maidservant, and half freewoman that was married to Reuven, and was freed, and married his brother Shimon, and the two of them died, and she came before Levi their brother, she does Yibum with him, for she is not the wife of two dead men; For if Reuven's marriage is valid, Shimon's marriage is as nothing, and if Shimon's marriage is valid, Reuven's marriage is as nothing.