Actually this is quite an obvious accident scenario.
But I have to think some while, until I can remember the last time a wheelchair out of control ran over some poor ladies.
(I can't help: I am quite sure, it has to be one of aliencaster's stories - but maybe I am wrong).
Too bad Lija's arms are casted. If she could use her arms properly she could steer to her right side to avoid the incoming impacts by breaking into the sand pile.
But maybe Sasha is a lucky girl. She could drop on Lija's lap, right before Scarlett drops on her lap. The three-women-sandwich wheelchair would speed up, because it is more heavier than before. Scarlett will be the unlucky girl who has to take most of the next impact. And it is likely this impact would be a hard one.
By the way: I really appreciate that you have given names to your characters to follow her stories easier!