The "health-related issue" Mary-Kate Olsen (search) is seeking treatment for is something she has struggled with for some time, her twin sister, Ashley, told People magazine.

Both People and Us Weekly have reported that Mary-Kate has an eating disorder (search). Her publicist, Michael Pagnotta, has said only that she entered a treatment facility recently "to seek professional help for a health-related issue."

"She's hanging in there," Ashley Olsen (search) said of her sister, adding family members were very appreciative of the public support they have received since Mary-Kate's problem was disclosed.

"The support's been really amazing," Ashley Olsen said.

She added that her sister had been struggling with the problem for "quite awhile" before the family stepped in and got help for her.

"She is taking the time she needs for herself right now," Ashley Olsen told People. "Right now is especially a good time right before school so she can get healthy."

The twins, whose most recent movie is "New York Minute," turned 18 earlier this month and plan to attend New York University in the fall.

Olsen, who recently canceled a trip to Australia to promote "New York Minute" to be with her sister, declined to describe their recent meetings.

"It's private," she said. "I'm here for her whenever she needs me, and, you know, that's how it works."