Someone told me that Brahma-vaivarta Purana & Krsna-jnama Khanda have this verse:
asvamedham gavalambham sannyasam palapaitrkam devarena sutopattim kalau panca vivarjayet
The above verse seems to mean: “Five things are forbidden in the age of Kali – horse-sacrifice, cow-sacrifice, acceptance of sannyasa, offering flesh to the forefathers and begetting a child in the womb of the wife of one’s elder brother.” Nevertheless, it's not known what Vedas say on this issue.
