I personally have not heard about a person being in a car accident as a result of high blood sugars. My own brother has been in several car accidents because of low blood sugars where the senses are impaired, and believe it or not the police thought he was drunk. I agree with you that it was the stress of the accident that made his blood sugar high. Have you ever noticed that stress caused him to have high blood sugars in any other situation? My son just this morning had a high blood sugar I believe caused from the stress of going to the doctor. I wish you the best.
Having a severe low can make a person appear to be drunk and do things that are not normal for them. When some people are high they also behave differently. I would recommend that your brother talk to his lawyer and his doctor, that he give them permission talk to each other about his condition. (Due to new HIPPA regulations he will need to give them both written permission.) Together they might be able to prove that it was related to diabetes.
Your brother could have been low at the time of the accident, but due to the stress have shot up to a high. Does your brother where identification showing the police that he is type 1? Hope all goes well. My prayers and thoughts are with your brother.