Very sorry that your family had to suffer this loss.
I've been through what you are experiencing with your SO being freaked out about this situation. Were it a freak accident, the argument against you might hold more water, but it was supposedly a case of lost control and a single-vehicle accident. Not to pass judgment on your BIL, but an accident investigator would probably place blame exclusively on him, likely for excessive speed or some such unsafe condition he chose to be in.
Only you can be the judge of how focused and safely you ride, and only you can convince a reasonable person that a similar fate does not await you, as you don't trust other drivers to do anything to stay out of your way, and you don't trust the road surface enough to push excessive speed through the corners.
You read all about every incident to date and contemplate others' recommendations for safer riding habits, right? You gear up with the finest equipment available every time you saddle up, and in the very-reduced chance that something untoward gets you and you are disabled or killed, you carry enough insurance to not leave your family facing financial ruin, don't you?
Yes, you (and all of us) need to seriously evaluate your abilities, riding style, and risk exposure, and then determine if you are comfortable enough to have the right attitude to return to the two wheel lifestyle.
Best wishes for a healed family and smooth recovery....