Tifere explained that she had no idea that her former husband was seeing Keys and found out months later.
She told Sister 2 Sister: "We were married, sleeping in the same bed and having sex every night while he was seeing Alicia. OK? It was an affair. 100%. I don't know why people want to change that rule so much.
"We had just had a kid, for crying out loud. Our child was 8 months. I found out about Alicia two days before my son turned 1. They had been doing their thing for months before that."
Keys and Beatz married last month and are expecting a baby together.
Tifrere added that Keys was trying to get her own son Kaseem Junior to call her "mummy".
She explained: "I asked her to not have my son thinking it was okay to call her 'umi'. You're not his umi. You just met my son. 'Umi' means 'mummy' in Arabic. I'm like no. I'm glad she's having her own child now because now she will have someone to call her 'umi'.
"Like, if you want to be a stepmother, that's great. Be a stepmother. But don't think that it's okay to make a kid call you 'umi' or 'mummy'."
