Updated

Lindsay may not be the only Lohan sister with serious issues.

Model little sis Ali, who signed a contract with Next Model management last year, is looking dangerously skinny, with experts estimating she weighs less than 100 lbs., and saying she may be on a serious starvation diet.

Friends close to Lohan tell Fox411.com they are starting to get worried about the drastic weight loss, and about Ali’s health.

“She just keeps getting smaller and smaller,” one pal told Fox411. “At what point will her parents tell her that enough is enough?”

In recent pictures the 18-year-old's breastbones are prominently sticking out of her chest, and her shoulder blades are protruding from her back. Her cheeks are sunken and there are dark circles underneath her eyes.

More On This...

PHOTOS: Ali Lohan looking super skinny.

“I would estimate her weigh to be under 100 pounds, which for any women of her height of 5'8 ½"  is clearly underweight,” nutritionist and eating strategist Rania Batayneh, MPH, told Fox411. “Despite her claims that she has a fast metabolism, we know that she has lost weight and it is frightening. To accomplish this type of weight loss resulting in her current physique, which appears void of muscle tone, could come from serious calorie restriction or from an eating disorder.”

[pullquote]

Batayneh does not treat Lohan.

Ali’s dad, Michael Lohan, tells Fox411.com that at first he was very concerned about his daughter’s drastic weight loss, but that he thinks that it is under control.

PHOTOS: Ali Lohan through the years.

“She isn’t doing it because it is a psychological issue. She is doing it because she wants to be a model,” Lohan said. “If I thought it was an issue, I would definitely say something. Ali and Lindsay live together, and I think Lindsay would say something to her if she thought something was wrong.”

Sources close to the family tell Fox411.com that the younger Lohan sister is currently planning a full-time move to New York City to try to make it as a model on the East Coast, a city where she will be competing with even thinner young women for potential modeling gigs.

Ali has been pinned to her big sister’s side throughout the past five years of Lindsay’s personal and legal dramas. Addiction experts tell us that siblings who are placed in high stress situations like the one that Ali has been in are more susceptible to attention-seeking behavior.

“We know from treating families at Caron that all family members suffer – not just the addicted individual [and] that families need to learn to handle the stress, disappointment and sadness around addictions in constructive ways,” Paul Hokemeyer, Phd, LMFT, Addictions Therapist with Caron Treatment Centers told Fox411.com.

“Siblings of people who suffer from addiction often feel like they are invisible because the parents attention is focused on the addicted child," he said. "Consciously or unconsciously the non-addicted sibling may begin to act in self-destructive ways to get parents attention and be assured they are loved.”

Hokemeyer does not treat Lohan.

Regardless of why she is doing it or whether it looks good in front of the camera, this kind of intense weight loss now could lead to serious damage to Ali’s body in the future.

“Downfalls of this type of behavior includes breaking down muscle (metabolically active tissue), losing hair, slowing down your metabolism and can even lead to heart failure,” Batayneh said.

Representatives for Dina and Ali Lohan did not return calls or emails for comment for this story.