05/15/2013 at 09:00 AM EDT
She was facing down the most frightening challenge of her life, but on Valentine's Day of this year, Angelina Jolie
was focused on nothing more than dinosaurs, bugs and precious family time.
When the star visited the Natural History Museum in Los Angeles on Feb. 14 with fiancé Brad Pitt
and their 4-year-old twins Vivienne and Knox, "the kids could barely contain themselves, but Brad and Angie spoke to them in quiet voices to calm them down," says an observer. "They stayed back and let the kids explore fossils and insects on their own. They all seemed to have fun."
Before Mom and Dad headed out for a romantic dinner later that evening, they gathered all six children together for a family gift exchange – the very sort of tradition Jolie's mom, Marcheline Bertrand
, delighted in fostering during Jolie's own childhood. "The older kids had made art projects at school that they gave Brad and Angie," says a family source. "Every special calendar day sparks a big celebration for them."
It would be the last such celebration before the surgery that Jolie would quietly undergo just two days later: On Feb. 16 – and with Pitt, 49, by her side – the 37-year-old Oscar winner underwent a preventive double mastectomy
at the Pink Lotus Breast Center in Beverly Hills. In a candid op-ed published in the New York Times
on May 14, the actress explained the gut-wrenching decision as a "proactive" choice that was "not easy" but "is one I am very happy I made."
Noting that her mother died of ovarian cancer
at age 56 and that she herself had tested positive for the "faulty" BRCA1 gene, Jolie – who kept up her globe-trotting humanitarian work with trips to the Congo and London during her ordeal – said the operation reduced her chances of developing breast cancer from 87 percent "to under 5 percent."
"I can tell my children that they don't need to fear they will lose me to breast cancer," she added.
Pitt, whom she praised for being with her "for every minute of the surgeries," offered his own words of support. Calling Jolie's choice "absolutely heroic,"
he added, "All I want is for her to have a long and healthy life, with myself and our children. This is a happy day for our family."
Pitt's mother Jane echoed that. "We're so very proud of Angie," she said. "This means so much to our family, especially our grandchildren We love her dearly."