I think Dean surmised that Charle's mom was kind because of what she told him- she read to her every night and was coming to get her at the sleepover when she was scared and wanted to come home. i don't think it's a stretch to say she was a kind mom from what she told of her.
That conversation with Charlie came AFTER he said all that since at the time he said it, Charlie was missing. Charlie told him all that while they were in her dream much later. And the nurse didn't say more than that she was injured in a car accident when picking up their daughter from a sleepover.
I know, I'm a bit nitpicky and it's not something that really bothered me, it just stuck out in an otherwise excellent episode, which for me, hit all the right buttons. I even liked Dean's developing relationship with Charlie as some sort of little sister and normally, Felicia Day's special brand of "quirkiness" makes me cringe and throw things at the TV.
More on my list of good things: Dean's disgusted face when he sees Sam stumbling out of bed and his equally disgusted face when he turned off bloody "Walking on Sunshine".
Sam's "This is stupid" when Dean makes him prove he can shoot straight.
The reversed "I love you - I know" exchange that normally goes the other way round for Charlie. Normally, she says the "I know" part.