This is a question I have been asking myself and it's hard to know, especially when we don't know how impaired she was and how bad she was feeling, but I am 99% sure I would have acted very similarly to Diane.
Once I get on the phone with my brother, If I am able to act like nothing is wrong and I didn't know why Emma had called then is my first choice. But as I talk to my brother, I piece things together - Emma's worried, she's gotten her father worried, I DO feel like hell and I am slurring a little. So I try to buy time.
I go to choice number 2 - I tell a little bit of the truth "Yes, I am feeling badly and confused and having some visual disturbances - I don't know what's wrong with me." This buys me some time because I am absolutely incapable of telling my brother who has entrusted me with his kids, who thinks I am an awesome mother and person and who doesn't know how much I drink the truth and I'm still thinking I can wiggle my way out of this - after all, I've been fooling him and everyone well for a while now.
Then, once I realize my brother iss coming, PANIC and the desire to buy some more time would rush over me. I would come up with a plan to get myself out of this.
I would leave the phone on the side of the rode because that works into the story I am planning to tell my brother. My plan at this point is to not drink anymore and to sober up in the ride to my brother's house. When I get to his home, I will call him on his cell phone and say, "OMG - I am so sorry - you must have been worried to death - but when we were talking earlier and I was feeling so terrible, I had gotten out of the car to get some fresh air and talk to you and then I flaked and left my phone there. I don't know what happened - I was feeling so bad and so weird, but then after I spoke with you, I started to feel better again, whatever it was passed - I think I just need to get home and rest - I will call the doctor tomorrow morning, blah blah blah"
By the time my brother gets back home to his house, I will have had time to brush my teeth, straighten up and really swing home with this story. I know my brother will buy my story because he has no reason whatsoever to think I would lie to him.
I realize my brother is coming for us and I can't be found, so I start looking for a different route to his house - a route that he wouldn't normally take. This is how I wind up on the Taconic. Once I'm there, I am completely focused on getting us home and sobering up and going over my story in my mind so I can put on the performance I need to put on. I have complete tunnel vision. Then - the collision.
Remember - my story isn't going to be perfect - there will be some holes - my brother may not understand my actions, BUT it will buy me the time I need to act sober again and it's the best plan I can come up with.