The funny thing is she spoke to every single person in family. My father, mother and myself and then specifically called later to tell my brother as he wasn’t available earlier. She said that she didn’t want him to feel left out which is really quite absurd.
It seems like sharing important information is a very special family tradition, at least in her understanding. Forgetting or failing to tell such an important bit of news with one person might make him feel left out. I have had that feeling when all my siblings and their children shared a bit of news but never bothered to tell me. For some reason, you don’t share her joy in this special event of your cousin growing up and getting his/her first car.
If it was not your cousin’s first car, or in any obvious way special, you could make conversation to find out what made this car purchase so special. Then, perhaps, you could share in her joy and it would not seem absurd that she made this effort to share the news with every member of the family.