Bad Obsessions

Hi, 

I'm worried about my sister. Lately she's been obsessed with fire. She talks about it, non stop facts about fires which was fine she's always loved sharing facts. But more recently she's been looking up fire pictures and videos. If she gets upset she says about burning things. I don't think she would ever do that but it is a worry and I have hidden the lighters and matches just in case.

She's 26 but still very childlike for her age. I will mention three years ago our sister was killed in a car crash and then she tried seeking support which sadly resulted in mental health teams trying to help her autism but they made it much worse and just stuck her in a hospital and that's made her worse and I'm sure it's behind her problems now. 

What can I do to help? My parents are at a loss and no one wants to involve any professionals again as we don't want to see her in a hospital. 

I'm not autistic but I do try to understand and I know she must be feeling rotten right now but I hope she knows I'm here for her. She's not talkative much but I do write to her as I know she finds it easier. I'm wondering if I should write to her about this but I don't want to push her in to a corner. 

Advice and suggestions are more than welcome.