I don't really know what to advise if I'm honest, but one of my good friends had a riding accident a couple of years ago whilst out on a hack. The 13.2hh pony she was riding (from the riding school) tripped in a pot-hole and she flew over his head, he then tripped again and landed on her. She has had a fractured spine for nearly two years now, was partially brain-damaged and is in pain pretty much all of the time. The doctors said that if she hadn't of been wearing her body protector, she wouldn't of been here today. When she looked at riding again and got the all clear from the doctors, she'd decided to go back to the riding school and start there. But the horse she LOVED and always rode before the accident, had been sold on to someone else. She found this horse, Chance, and got on him again. She did trot on him, but canter was too bouncy for her pelvis to handle (as it had shattered in the fall). But where any other rider who's been riding for most her life can just throw their leg over the saddle and be off, it took her about 5 minutes to dismount from him and that was 5minutes of alot of pain. The doctors have said that she may not be able to ride again soon as her spine is worse off than they originally thought. She may get down and upset by it, but she's more determined than ever to ride again. She's got this year planned and Chance's new owner is more than happy to let her ride him when she wants to. She's even planning to take him Cross-Country jumping this year! She proves that no matter what happens, if its what you love - just stay strong and go for it. I'm sure your oldest niece, Leanna, will be fine and ready to ride within time. Don't pressure her or force her to do things she's not ready for, maybe start her with groundwork with the horse that was involved in the accident - it might increase the trust between them? And when she's ready, start of with just a walk whilst riding him perhaps? And have someone walking next to them just to reasure her.
Hope it helps, and hope she's riding soon! X