The 'Scream & Shout' hitmaker praised his fellow singer for taking the leap from being a musician to starting a family with her actor husband, Josh Duhamel, reported Contactmusic.
"(I'm proud of her) for taking that leap. You know, our life... We travel, been doing crazy things, going to different countries and selling out stadiums. So, to step away from that and to do a family, that's a big leap. She's going to be an amazing mum.
"I remember on our last tour, she was talking about how her and Josh want to have a family. That's a dope thing to think about," he said.
"That's like incest. Fergie's my sister. I'm really proud of her. Josh Duhamel's amazing," he added.