Sending a letter-- thoughts?!?

Of course your aunt is upset at the loss of her dog, but you expressing sympathy isn't going to make her feel worse. If anything, she'll be comforted that you're thinking of her. I understand the point your step-mother is trying to make, but ignoring the loss won't make it better. Look at it this way: If it had been a person close to your aunt who passed away, would you wait to send a letter or card until she "felt better?" No. Send the card. Your aunt is lucky to have you for a niece