I'm starting to think that the person who died wasn't just her brother, but maybe also their child...that would explain why Kim Shin would fall in a sudden depression and give up on everything. Then that would mean that Old Mi Rae is actually not so much as blaming him but also protecting him from the pain. It would actually make sense. So my theory :
On their anniversary (or some other occasion), her brother takes their child in order to let the couple get some privacy, and as they leave a car hit them and kill them on the spot. They witnessed all that, and are traumatised for life. At first she tries to comfort him but nothing works, then like many parents who lost a child they start blaming each other and when they are finally going to split up, Mi-Rae make the ultimate sacrifice "it would be much better if we never met" and go back in time to change everything.
What do you think ?