While filming a new reality show abroad, Brown, 34, collapsed and was airlifted to New York, where she underwent emergency surgery on Sunday.
"Doctors found one tumor blocking her colon completely so they had to do emergency surgery that evening to remove the blockage," Brown's sister Megan tells PEOPLE. "They were able to remove the mass, but once they were inside they found out there were multiple tumors."
Brown then underwent a second emergency surgery three days later when complications from her first operation caused an infection in her abdomen. She is currently in the ICU at a New York City hospital in stable condition but closely monitored by doctors. She's expected to stay for about two weeks.
"The second surgery was two steps back," says Megan. "But hopefully in the next few weeks she'll be well enough to start chemo."
"She's in and out," Megan adds. "She's on a lot of medication but every once and awhile you'll see her spirit, the light back in her eyes."
It's her spirit that inspired fans throughout her tumultuous battle with cancer over the years. Brown, who first found fame on MTV's Real World/Road Rules Challenge, was diagnosed with ovarian cancer at age 23 and had to have one ovary, several lymph nodes and part of her fallopian tube removed.
When she found out the ovarian cancer had returned in 2012, she shared the emotional and often painful journey to recovery in a blog for PEOPLE.com.
Now "she's okay with people knowing where she is," says Megan. "She's had an unbelievable outpouring of support. People will text me and I'll read her their messages while she's in bed."
For those who want to show their support from afar, Brown's family has also set up a support page for the reality star on MedGift, a gift registry Brown started to help those suffering from any illness.
"Cancer has changed who I am," Brown told PEOPLE earlier this month. "It sounds like a cliché, but I've only got this one life, and I need to live every day to the fullest. I'm a lot more appreciative of things now."
While Brown has a long road ahead of her, "she's such a fighter. She's tough as nails," says Megan. "This is the third time she's had to deal with this and she's ready for it. It's definitely an uphill battle, but I know if anybody can do it it's her."