What a tragic accident. I am so very sorry for your loss.
Please give your brother a big hug.
He must feel so guilty, but you are right...it was a complete accident. Thank you for sharing your story with us. There are so many times that I do this EXACT same thing....tell my doggies to go potty while I'm putting stuff in my car and then I lean down to pick them up when they are done. I will know to be extra careful from now on.
Rest in Peace, sweet angel, Isaac.