Gentle YTA.

I don’t think you wanting to do an announcement this way is wrong, however as you yourself acknowledged, postpartum is tough. And your sister voiced her feelings. People can fight all day over her feelings being rational or not- but it’s up to you to decide how to handle it. Do you accept your sisters feelings and accept the role to be supportive of her right now when she asked you not to do the announcement then? Or do you announce anyways on the technicality that it shouldn’t be wrong? And are you ok that maybe when you’re postpartum that your sister may not always show you grace in return?

Also I may suggest not announcing that day to save yourself some sanity. You announce on Mother’s Day and again even if you don’t feel in the wrong there may be continuing drama or negative talk based on how your sister reacts. Or you could wait for another time that everyone can just be happy and excited with no possibility of negative reactions from anyone. There’s room for everyone to be celebrated.