Everything played in her favor regarding the accident verdict. She had no history of criminal behavior, was from a good family, and brother and sister got along well by all appearances. In the courtroom, pre-makeover, she looked like a trustworthy librarian, somebody who'd never shoot their older brother on purpose. So it had to be a mistake, a tragedy instigated by a sleepy, drugged young woman after her brother clumsily lost his keys in the dark.
Although if someone had seen any reason to question him, Charles might have been able to tell them about the strain between Beth and Bryce Or at least a little about it. Though it's pretty likely he wouldn't have really wanted to, since the few times you see him he strikes you as someone who wants to mind his own business.
(I always sympathize with characters like Charles - people are always making sure I hear all about their personal lives, and everyone else's.)
Could she have got away with shooting someone three times? She didn't fire a warning shot, she didn't say I have got a gun here don't move. She just fired straight away.
Yes, but you're looking at it from a "viewer" point of view.
Remember, there were no witnesses to what she said or did, so she could say she said she had a gun, etc.