Credit: Bauer Griffin

Ali Lohan needs a snack.

Days after the 17-year-old model stepped out in Beverly Hills with a shockingly skinny frame and gaunt, pale face, she joined big sister Lindsay on the Hawaiian island of Oahu on Friday.

For this outing, the younger Lohan (who now goes by Alianna) looked just as slim in a midriff-baring tank top and tiny short-shorts.

Ali and Lindsay, 25, were in Hawaii to check out the Pipeline Masters surfing competition.

It's a big week for the Lohan girls: As many fans worried about fledgling model Ali's increasingly tiny frame, Lindsay's talked-about nude Playboy pictorial (aping Marilyn Monroe) was leaked online weeks before the mag planned its debut, thanks to a clever Russian hacker.

Ali, who turns 18 later this month, recently signed a multi-year contract with Next Model Management, and has denied claims that she underwent plastic surgery.