J.S.
If she was sleeping through the night before, it could be a couple of things. One is a growth spurt which means she is really burning up the calories and needs more food than usual for a while. However, it doesn't really have to be at night. Try giving her a top-up before bed. If she is on solids make sure this top-up includes some cereal to help hold her over until morning.
You say you are only breastfeeding her during the night now. Is she bottle fed during the day? The other thing could be that she's figured out that if she cries at night she gets a nice one-on-one cuddle with mommy. This is a hard one. Obviously you are working during the day and have another child that also requires attention. Is it possible to carve out a little time each evening and morning with just her? Even just 10 mins at these times could make a huge difference.
Whatever you figure out is causing it, the night time solution is the same. Don't feed her at night. Get dad to go calm her when she wakes. You may need to sleep in another room for a few nights (I know!), but it will make all the difference. If she can't see or smell you she will give up. Dad should not turn on lights or talk too much. Just Sshhhh, or 'your ok', then cover her back up and get back in his own bed. Waiting at least 10 mins before getting back up to calm her. I realize this will make for some difficult nights, but in just 3-4 days she should be back to sleeping through. Start with a Thursday night (only one work to day to get through without much sleep). Take advantage of a nap on Sat and Sun when the kids nap to help keep your sanity. If you stick to the same response (dad responding) everytime she wakes, she will soon settle back down.
You'll get through this!