Well we are doing our thank you cards today.
The holidays and busyness is now over, I think.
To me, this is still "prompt" enough.
Its only been about 1 week since Christmas day.
Your sister, is being a bit bossy about it all.
She is not your kids' parent.
Some people.... use gifts and their ability to gift "expensive" things... as a power trip over other people.
They are the ones... that "chooses" the expensive gifts they are gifting to someone... but then, they "attach" their own "rules" about it and expectations about it and upon the recipient. So then, their "gift" becomes not a pleasant situation. And they use it as a way to criticize, the recipient. Passive aggressive and bully like actions, results upon... the gift recipient. Then, it puts the recipient in a bad position, of having to.... "please" the gift giver. Of that expensive present etc.
Vicious cycle ensues.
Perhaps, instead of texting her yourself, the kids, could have called her to thank her, too. But again, you wouldn't know to do this... because no one can read... your sister's mind and know what HER expectations are, that she attached to those gifts she gave your children.
So again, this turns into a misunderstanding and an unpleasant, thing.
Then, your sister will probably hang it over your heads, and bring it up again and again about what bad manners they have etc. and how you are not teaching them well etc.
I hope you told her you had already mailed the thank you cards to her??????????
It is just not according to HER timeline of when it should have been done.
And then again, no one can please... people like this, perfectly enough.
They are NEVER, pleased.
They are controlling and on a power trip.
I have a sibling like that.