I can't decide if I am being unreasonable or not, so thought I'd don the hard hat and ask. If you were to buy a gift for a 5 year old when you were on holiday, would you buy the child's younger sibling 2.5 years something too?
Am I being unreasonable to think that you should get something for each. I wouldn't expect to buy something for both on birthdays. I just think a random gift from a trip is different.