With regards to online support for young carers, it's quite lacking unfortunately. There were forums set up I think at some stage but I'm fairly sure they're very quiet.
Again, there's a website but it does lack a bit. The forum's attached to the website, I can't link on here but it's youngcarers.net.
She might be better off looking into more general chat for young people, I'm thinking something like The Student Room. There's certainly other young people on there who take on a caring role within the family.
If she's a Facebook user she might find something on there too, there's bound to be a page of sorts. Some are better than others.
You could also check with your local authority - most if not all will have something. Some groups are very active, some not so. Some will offer group activities - in my experience, we had a larger programme for non term time, but some city based groups will meet in evenings and weekends. She might find she can get some 1-1 support which can be very helpful indeed.
It's worth talking to your ex's family if at all possible. You need a clear idea of what's going on in order to best support your children. If he's in a care home or community hospital, etc, there might well be all sorts of support available through them, even though she's some distance away.
It's also a good idea to encourage her to let her school know, like you if they're clear on what's going on they're in the best possible position to support her fully through this.
Don't forget as well that there's a plethora of organisations and individuals who will be able to offering formal or informal counselling. This includes churches, youth organisations, charities, helplines, proper 1-1 or group counselling sessions.. Your GP or school nursing team should know more.
I'm torn as to whether there's other stuff out there. I have always wanted to write a book as there's just nothing. I've read a couple of Jacqueline Wilsons (Illustrated Mum is good) and one regarding an older (early-mid twenties) woman who was dealing with the challenges of a disabled sister and unwell mum (Look the World in the Eye - But Don't Stare, it's a
Rude).. But there's not much at all. There's stuff directed at Alzheimer's of course but I feel it's either going to be targeted at very young children or adults, and neither are probably very appropriate.
All my love to your daughter (and your son, who will be facing different but very difficult challenges, and not forgetting you either) x
(I'm happy to yap with either via here if you /she wishes at any time - I am a carer to my mum and sister, watched my gt gran suffer vascular dementia and now watching my gran decline slowly with Alzheimer's. Also have big problems with my dad. So can empathise to some extent).