Julianne Hough: Family Blonding Turns into Puke Fest

Julianne Hough and her 3 sisters are having a kick ass Mexican vacation -- bikinis, sun, cervezas and ... projectile vomiting off a boat!

The former "Dancing with the Stars" judge and pro is down in Cabo San Lucas on a family girls trip -- Mom's there too -- and they all set sail Tuesday. Looked like a ton of fun at first ... and then las hermanas y madre turned their stomachs inside out.

Unclear if the illness was booze or wave induced, but Julianne comforted everyone by making sure to capture their puking on her iPhone.

Viva Mexico!