The couple, pictured at the Metropolitan Museum Ball together, have a total of nine children between them.
The 29-year-old supermodel has two children – one from each of her first two marriages.
Rohan has seven children – two with his first wife Geraldine Khawly and five with singer Lauryn Hill.
Both Rohan and his fiancée have sons called Zion – Lauryn Hill wrote a song To Zion about her first child with Rohan.
Isabeli said her husband-to-be wanted to get hitched in Ethiopa, where the Rastifarian movement began.
She told Brazil's Veja magazine: "Rohan told me we have to return to the roots so the marriage will last forever."
The brunette beauty has recently been seen in H&M's new campaign (below) and has previously modelled for Sports Illustrated and Victoria's Secret.
She was named as one of the world's top earning supermodels by Forbes in 2008.
The couple met last year after Rohan, 39, separated from former Fugees singer Lauryn.
It then emerged that the 37-year-old songstress was pregnant by someone else.
Lauryn has rarely discussed their long-term relationship. But after much speculation following the birth of her little boy she posted a message on her website defending Rohan, her partner of 15 years.
She added that they've had a "long and complex history" but love their five children together.
The couple are parents to Zion, 15, Selah, 14, Joshua, ten, John, nine and four-year-old Sarah.
Lauryn often referred to him as her husband, but it's unclear if they ever married.
Lauryn Hill confirms that her partner of 15 years is not the father of new baby
Sacha Baron Cohen spotted canoodling with George's ex Elisabetta