Do you think it might be the thought of the baby coming along? Have you spoken to her about it?
Tia can be quite aggressive at times, but she has been more so, since I've gotten pregnant.
She seems to be very excited about the new baby, but apprehensive at the same time... So she's been quite moody and sullen and difficult at times... Once I sit her down and chat about her feelings she's ok for a few days... but like you said it goes back to the way it was..
I think it's harder when they are older... especially if its a second relationship baby.... They know they aren't being replaced, loved less etc... but they are still children, and they just don't seem to be able to grasp the concept well... so they act out... the chats give them time alone with you (which they want) and to get things off their chests... after all most people I know get moody and depressed when they have something they are keeping bottled up inside.
Just a thought..