Only if he is dead

Leviticus 20:21 says, “And if a man shall take his brother's wife, it is an unclean thing: he hath uncovered his brother's nakedness; they shall be childless.”  It must be ok if god likks your brother first.  Genesis 38:8 “And Judah said unto Onan, Go in unto thy brother's wife, and marry her, and raise up seed to thy brother.

38:9 And Onan knew that the seed should not be his; and it came to pass, when he went in unto his brother's wife, that he spilled it on the ground, lest that he should give seed to his brother.

38:10 And the thing which he did displeased the LORD: wherefore he slew him also.”  God got mad but I thought it was unclean?  God should make up his mind or clarify this rule.