What an awful situation for all involved.
It sounds like you're doing all the right things though.
Don't under estimate how important what you're doing for the children is. Providing love, care, honesty and stability as you are doing is vital.
It must be such a difficult time as the prognosis is unknown - it's impossible currently to make any firm plans or reassure the children as to a future that's unknown.
Realistically all you can do it provide a loving home environment that gives them a sense of security regardless of the outcome.
I'd also agree that your DP taking time off work right now is fruitless. Aside from the fact you have no idea how long she will be in hospital, his ex has her family to support her, you are doing a great job with the children and being brutal he needs to ensure he can be available to his children in the event of the worst case.
Obviously everyone is wishing for the best possible outcome wrt a full recovery, but I think you need to be realistic about timelines.
Brain injury is so hard to predict wrt outcomes and length of rehabilitation, so even in the hopeful event of full (or nearly full) recovery it's unlikely their Mum is going to be in a position to physically parent them for a long time (and it's in both hers and the children's interests that she can focus on rehabilitation and recovery).
Personally at this point I'd be assuming the children will be with you full time for at least 6 months to a year and researching/adapting to the implications of that.
Keeping their mums family in the loop and open access (as you are doing) is absolutely the right thing to do.
Unfortunately, it's a situation where it's hard to be proactive. You simply have to react to whatever is thrown at you, but I'd echo PP's who suggested getting in touch with children's and brain injury charities who have the experience to help you, your DP and the children navigate what lies ahead and where needed explain and medical information.