What's your opinion on this? Last year, after our annual extended family (step siblings and a cousin and their families, I all see about once a year) holiday get together, my mom kept asking if my kids sent thank you notes to the aunts and uncles (my step siblings). My son got a little annoyed about being told to send thank you notes and I tend to agree with him. He (and my daughter) both said thank you when they opened the gift and then said thank you again and a hug to each person, when it was time to leave the party. I really didn't feel it necessary for them to either send an email or handwritten note thanking them. Sure, it wouldn't hurt anything, but on the other hand their kids don't send me a thank you note for my gift to them and I don't expect them to, since I was there to get a thank you in person.
I have a feeling this is going to come up again this year with my mom (though I'm sure she isn't telling my step siblings to have their kids send me a thank you). I think if it does I will just tell her that I really don't think it's necessary since my kids thanked them twice already, in person.