Sunnydisposition raises a very good point.
In Hampshire where I live they have "Young Carers" groups, for children who have someone with special needs in their family. I wish they'd had these when my son was younger, as my eldest son missed out on so many things.
I think Barnardos also run young carer groups, and annual holidays.
Have a look and see what is available in your area. It might help them to talk about how they feel and give you a break too.