The 357th prohibition is that other1 men are prohibited from having relations with a yevamah2 while she is still waiting for the yavam.3
The source of this commandment is G‑d's statement,4 "The wife of the deceased is not allowed to marry an outsider."
Both the woman and the man are punished by lashes for transgressing this prohibition.
The details of this mitzvah are explained in tractate Yevamos.
Other than the yavam, who is commanded to marry her.
See note above (P217).
I.e. she is still in an intermediate status: they have not become man and wife through yibum, and they have not broken the relationship through chalitzah.
Although the brother-in-law has not given any indication that he wishes to marry her, the brother's death nevertheless creates an automatic connection between them similar to marriage. Therefore, she is not free to remarry until this relationship is broken.
Deut. 25:5.
