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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.

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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.

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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.

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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.