Phone call as the catalyst?

We know Diane was drinking and driving aggressively before her niece called her parents and made the infamous statement, "There's something wrong with Aunt Diane." What we don't know is if Diane had already decided to do something drastic. Up to that point, she may have been having a psychotic break, coupled with drunkennes; but never consciously decided she would end it all. After the conversation, she may have flipped out -- knowing that her brother was on his way. Being a person who is always in control, she must have felt panicked at the idea that her brother was going to arrive and find her in a most out-of-control state of mind. Without thinking, she threw the phone and made the split-second decision to drive head-on into traffic, without really any pre-meditation. If this scenario is correct, then she was in a psychotic state of mind and not thinking of anyone at all. That would account for all the questions about why she would kill her nieces. She wasn't in her right state of mind.