Mashonda Tifrere - who was married to the music producer for four years until 2007 - has branded the couple's relationship an "affair" and admits their romance had been going on for several months before she found out.
She explained: "We were married. Sleeping in the same bed and having sex every night while he was seeing Alicia. OK? It was an affair. 100 per cent. 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 eight months. I found out about Alicia two days before my son turned one. They had been doing their thing for months before that."
The 'Empire State of Mind' hitmaker - who tied the knot with Swizz Beatz on the French island of Corsica last month - is six months pregnant with the couple's first child.
However, Mashonda has revealed this hasn't stopped the singer from trying to persuade her son Kaseem Junior, now three, to call her 'mommy' or 'umi'.
She added to Sister to Sister magazine: "I asked her to not have my son thinking it was OK to call her 'umi'. You're not his umi. You just met my son. 'Umi' means 'mommy' 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 'mommy'."