Mabel wasn't aware that she would never see Dipper again.
She didn't have to worry about him. They were twins through thick and thin, and no matter what happened, would always somehow find each other in the end. She knew for a fact that no matter what stupid things she did or said, he would always, always, come back to keep her safe. She also knew that even though she wasn't the best twin in the world, and she forgot to respect Dipper, that he loved her. It was just how Dipper was.
It was also why Dipper had gone.
Dipper was many things and protecting his family was one of them. Mabel was aware that this part of his character had led to a lot of sticky situations but she also knew that Dipper was pretty damn lucky.
But, luck had to run out eventually.
Everything would be okay because Dipper always found out a way out of sticky situations.
As years passed it became exceedingly clear that Dipper would most likely never be seen again, and Mabel couldn't help but feel that it was all her fault.
/¤\