I have a cousin who doesn't seem to know the word "thank you". It gets on my tits. We buy for each others children (and it will be something she specifically wants as I asked beforehand and she does me) and I can physically hand the present over and she doesn't say thank you. I've passed stuff along, nothing. She got married this year, we all went, cash was given, as requested, I assumed for something like this we would definitely get a thank you in some form (I'm not fussy, a text would do) but nope, nothing at all. I find it so rude tbh.
I remember her being the same as a child and not being told to thank the sender of a present but I assumed as an adult, you kind of catch on that this is the done thing. Especially with something like a wedding!
She's not a horrible person or anything but this one thing really grates on me and it's just such bad manners.