When my second daughter was born, my oldest daughter was 19 months old. She had been potty trained already for at least two months by the time of her sister's birth. After bringing home the baby, however, she would pull her pants DOWN whenever I fed the baby, and poo right there on the spot in front of me! It was pretty obvious what it was all about. I think the same thing is what is happening with your daughter. It is sibling rivalry at its earliest stages. The way i handled it was by paying her more attention whenever baby was awake, and also having her have more playtime with children who were completely potty trained. I did not push it on her, did not scold her, etc., praised her highly whenever she went potty, and within a month, she was back to being completely trained. A friend of mine said her older daughter did things like dumping baby powder all over the new baby, pulling its hair, etc., so I figure expressing themselves by reverting back to being a "baby" beat those methods of expression at least! Right now, your daughter is expressing her desire to be the center of attention (a normal desire for a two year old), and her displeasure at having been displaced in your sister's life by this new baby. She will get over it once she understands that her place in this world has not been displaced by this baby and when she figures it all out, which she will, she will be fine.