Wade, who has two sons, Zaire, 11, and Zion, 6, got the boys involved in the big moment.
"They wanted to catch me off guard. We were just spending what I thought to be a fun day with the kids, having a little quality time," Union said of the December 21 proposal. "We went to brunch, and we went to tour our house, which is being constructed. It's a construction site -- it's nowhere close to being done."
"I should have known something was up when [Wade] was like, 'Are you gonna do your hair? Are you gonna put on makeup?' I'm like, 'To go to a construction site? No,'" she said.
"They were like, 'We wanna do a presentation for you.' They like to do skits and songs and stuff, so we turn around, and they're like, 'Okay, we're ready!' and I turn back around and all the boys were holding up signs that said, 'Will you marry us?'"
Union said the moment caught her off guard.
"They've been asking us to get married for two years and they talk about it all the time, so I'm thinking they've just made signs," she said. "I was like, 'Well this is awkward.' I turn to look at D like, 'Ugh, they just made some signs,' and he's down on one knee with a ring and said, 'Will you marry us?' And I was like, 'Oh, this is serious! This is a thing! Yes! Of course, yes!'"
Both Union and Wade shared a photo of the stunning 8.5-carat ring, designed by Jason Arasheben of Jason Beverly Hills.
"Dwayne Wade was 100 percent involved in the process," Arasheben said. "I made three personal visits to Miami to present different options until they found the perfect one. Wade picked out the stone and they made the ring custom."
Wade, 31, and Union, 41, have been dating since 2009. This will be the second marriage for them both.