How do I unmat incredibly matted hair?

My little sister hasn't been brushing her hair - she can't tell me for how long. She's 10 years old. She starts school again tomorrow, and I've been granted the task of unmatting her hair.

I've been unmatting it for years - hours of brushing and sperating and eventually a full on scrubbing wash once most large knots are out, but this is probably the worst I've seen of her matting. I can barely separate the front, and can't pull out even the smallest sections. Her hair is maybe 2a.

It's already 7:30pm, and I would really like a way to properly separate this hair, because currently it's a pillow.

Part of the fight here is getting my mother NOT to put my sister in the shower. Wet hair would make it nearly impossible.

Any tips?