As @EmpressaurusKitty said, ExH probably made her promise not to tell you (possibly because he knew that would drive you mad), so she's in that awful position of trying not to take sides. Doesn't by any means suggest he comes before you, but I'd guess you brought her up "proper like" so if she's made a promise to him she won't want to break it. Plus of course if she tells you and he finds out, that could ruin the relationship and though you don't give a flying one if he gets hurt by that, you would care if it made DD2 unhappy and feeling guilt ridden.
Somewhere deep in the back of your mind is the idea that as he was the one who wronged you, he has no right to have secrets or even a good relationship with the children. I can completely understand that but where it doesn't directly impact you, you don't really have that right. Yes it's impacted you indirectly by him not being available e.g. when your DS first lost the use of his legs, but you were the one DS went to for help not him despite DS knowing ExH has the car not you.
To turn your rage into something more enjoyable & productive, look on his trying to keep you in the dark as very childish behaviour. That's the only power he has over you, how absolutely pathetic, weak and inadequate he is!