question about Will's parents

Hi,

So Will's parents really died in a train crash? Was the only fantasy that Will wanted to believe they were going to a matinee of My Fair Lady, but in reality they were going for radiation treatment for his mother's cancer?

But I'm confused because, according to his younger brother's speech (below), Will is the person who told his younger brother about the cancer. Will's parents died when Will was 10 years old and Will is the older brother. In the scene below, the brother tells Will's friends that he found out about the cancer because 'he told me when i was 11.'

"He's sick and someone's gotta do something. Just look at him, who is he talking to half the time? You know what he thinks about his mommy and daddy? He thinks they went to a Broadway show on Thursday afternoon. A matinee on a Thursday? That doesn't happen. You ever seen my mother without a cigarette in her hand? My Fair Lady? No way, Dad was taking her to radiation. Breast, lung and liver cancer. And how do I know this? I know because he told me....when I was 11. He wants to believe in this fairy tale, you know? Our beautiful mother, our handsome father, freaking prince charming, cinderella. Matinee my ass. Everything was as bad as it could get, and then it got worse."

Or was Will trying to block out the cancer and Will only told his brother about it once in a moment of clarity?

The scene with the younger brother's speech seems significant, but i'm confused about it.

Thanks