Absolutely fascinating. You live and learn. I had no idea that there were places/groups where it was completely fine not to send a thank you letter. That explains my BIL then...he's not rude, just has different mores.
Amongst my family - and that of my DH - it would be the height of bad manners to fail to write a thank you letter after receiving a gift, regardless of whether or not you were with the person at the time. It is important afterwards to sit down and write a letter (hand-written is the only acceptable format) thanking the person for the gift and making some comment about why you appreciate it.
From the moment dot my kids have been taught to do the same thing - when they were tiny we obviously wrote them, and once they could write they started by signing their names. Now my 7 year old writes a 'shell letter' saying 'Dear....thank you for the lovely.... I really like it because....' which I photocopy and he then writes in each person's name, the gift and why he likes it. Usually no more than 'it's cool' but I know that relatives really appreciate it. My 11 and 14 year olds will sit down after a birthday or Christmas and write all their letters over a period of a few days without much chasing at all. So even after a party where they have received 25 gifts they will sit and write 25 letters. Well, mnore like 40 by the time they have done relations, godparents etc. I know no other way - I couldnt not do it, I would feel awful.
I really appreciate receiving these letters in turn, though confess I never notice whether I get them after we've given gifts for the many birthday parties the dcs go to - there are just too many. But where it's a personal friend or relation I would be a bit if I didnt eventually receive a letter.
And I always send a card saying thank you after lunch, dinner or a party.
Yes, I am personally keeping the Royal Mail in business !