Julianne Hough's older brother, 'Dancing With the Stars' pro dancer Derek Hough, says his little sis is doing just fine after her surprise breakup with Chuck Wicks ... and confesses he's actually not all that surprised about their split.

"She's 21 years old and her career is skyrocketing right now -- and it's tough to sustain a relationship, especially with somebody who's so much older than her," Derek told PEOPLE last week at Diddy's 40th birthday bash in New York.

Age difference or no, Derek assures that he, Chuck (who's 30) and Julianne remain friends and still enjoy each others company, when their schedules allow.

"We'll all still hang out," says Derek. "We're all very close. And (the split) is amicable. They're still really great friends, and that's the way to do it. They're both good. He's a great guy -- he's cool."

Julianne and Chuck met when both were opening acts on Brad Paisley's 2008 summer tour. They dated a year and a half before cooling their romance earlier this month.