DD (age nearly 4) has been invited to the birthday party of one of her nursery friends. It is a joint party for DD's friend (will be 4) and DD's friend's sister (will be 3) whose birthdays are only days apart. DD is only friends with one of the girls but I would feel mean getting a present for one sister and not the other. AIBU to buy a slightly more expensive present and address it to both girls or should I buy them one each?