Dear wrinklytum. Know where you are coming from. I really try not to think about it. My son is my only child so has no siblings to look after him when we go. Have an agreement with my brother and also my best friend who are going to look after him if i die. I try not to think about it, but sometimes you just cant help yourself. Trouble is you can worry about it but i think you should live each day and not think to hard about it. Have you made a will. If you really worry about it, put in your will what you want to happen to your child. Has dd got god parents or siblings. Maybe you need to look into independent living schemes in your area to put your mind at rest.
My sis and i have an agreement about taking on each others dcs, but I have said that I wouldn't expect her to take dd2 unconditionally if she thought she couldn't cope - I realise that she'd have to go to boarding school or part foster or something. That is being realistic.