Did your sister know it was a special dessert for you that you paid for? Or did she think she was just grabbing some dessert from her mom's house, which presumably is normal and welcome. Could your mom have even offered your sister some?

YTA for blowing up at someone over a dessert. It's a dessert. It has nothing to do with your sister being pregnant or not. It's about you not just being a bit disappointed to see half of your huge family size dessert is gone, but because you then went so far as to call your sister and gripe to her about it. And who knows what you actually said to her on the phone or the tone of voice in which you said it. I doubt you were being polite.