She is asking for as much attention as the 9yr old gets and the Baby, When she wont wear the clothes dont make an issue of it, back off ,if she gets a response that is what she wants, you are rewarding her behavior, ignore it and does it really matter what she wears.Yup I have heard the sing song thing its infuriating, ignore it,In fact the more you ignore her bad bahavior the more it will lessen, Good Luck.
I'm trying to picture this whole scenario. A couple thoughts come to mind:
1. There is a theory that siblings are like plants that are growing out of the same hole in the ground, growing toward the sun. If one plant grows extremely straight and tall, the next plant has to grow way to the side to receive the sunlight. In other words, the straighter the first child grows, the more crooked the second one will grow to get her parents attention as an individual. Is that what's happening here? Is your first daughter truly a golden child? If so, your wife needs to pick some characterstic about your 2nd daughter and really emphasize it. Is she a good runner? Does she draw well? Something that will stand out and be her characteristic, not just the sideways one.
2. I heard an interesting parenting tip about a younger child antagonizing an older child, and the solution was great. Tell your older daughter IN PRIVATE, THIS MUST BE A SECRET, that for every time her sister antagonizes her and the older sister doesn't respond, she gets $1. Or some amount. She just has to not react, and keep going on about her business without reacting at all. The antagonizing will get worse at first but should die out pretty quickly. She's doing it for the reaction obviously.
3. I'm curious that your wife called you about that singsong thing. I'm trying to picture it, and picture the power dynamic in your home - and I really can't imagine calling my husband if I had an ongoing disrespect/physical attack problem and this little thing happened. It sounds like you are perceived as the powerful one, and your wife is kind of just a conduit.
4. Prayers your baby that is on the way is a boy! You may have enough girl crazies in your house. ;D
Best wishes.