Starring as Meadow on the hit HBO series, The Sopranos, the actress naturally grew close to the late actor James Gandolfini, who played her father.
And after she was diagnosed at age 20 with multiple sclerosis (an autoimmune disease, also known as MS, that impacts the central nervous system), Sigler was able to keep her condition a secret for several years until symptoms started to present themselves.
"I was still able to hide it really well," she tells PEOPLE in this week's issue. "I didn't really need help necessarily, it was more like some pain and feeling uncomfortable but I could easily hide it."
The actress eventually told a few cast members, including Gandolfini, because "they were very much like family to me," she says. "You can only handle so much by yourself. So the people I felt safest with, I confided in."
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The support meant everything to Sigler, 34, who now has weakness in her right side and can't walk for long periods of time without resting. "Sometimes it makes you feel better to know that somebody else knows," she says. "And if you are having a hard day, if you're a little tired, even a pat on the back can make all the difference."
For more on living with MS and to contribute to the fight against the disease, visit The National Multiple Sclerosis Society
Having recently married her fiancé of almost three years, pro baseball player Cutter Dykstra, 26, Sigler opened up about her MS in this week's issue of PEOPLE.
The actress says she wanted to reveal her diagnosis now in hopes of helping others: "I'm never going to know why I have this until I live my truth," she says. "But it doesn't define me."