Another one here living in a parallel universe, then, because I don't think YABU.
I don't think it's too much to expect a close family member to make a note of the date of a big operation & express a bit of concern. My FIL recently had a very routine operation: still sent a text checking all was well. I thought that was fairly normal?
Having said that, OP, if you actually used the words "How come you didn't acknowledge dd's op." to your sister, I think it was probably inevitable that you were going to get a defensive response. It does sound accusatory, to be honest.
It might have been better just to say: "By the way, dd is recovering well from her op." or something. That would have left the way much more open for your sister to express concern or even apologize for not having been in touch, without feeling that she was being taken to task for forgetting.
Leaving the op. aside, I don't get the point of buying someone a present & then demanding that they visit you at a specified time to pick it up. That just creates hassle for the recipient / their parents, which seems at odds with the purpose of a gift.
Would be bad enough if the gift was for a recent birthday, but as it's now 2 months since Christmas, I would probably just tell your sister that you're really busy & that she'll need to post the present or just to hang on to it until you next see each other.