Hi K.,
I understand what you are going through, and how tough it can be. I have a two year old and a four month old (both girls) right now, and the sleeping patterns are a bit out of track for all of us in the house.
Two months before the new baby arrived, my oldest stopped sleeping through the night (putting her down was a challenge to say the least, and she would wake up two or three times in the middle of the night, and for good at 6.00 or 7.00am). By the time the new baby came we were exhausted parents already.
What worked for us (finally! and after a lot of trial and error!):
I created a "good sleeper rules and rewards" chart for her. I literally wrote the rules and we read them to her every night:
Good sleepers...
-Lie down in bed
-Close eyes
-Stay very still and sleep
-If they wake up and it's still dark, they sleep some more
Then we would lie down and do our nightly routine (milk, teeth, story, song, prayer, wrap in blanket, kiss). After that, I tell her that we are going to leave her door open, but mommy will be right outside the door. If she gets out of bed (she is out of the crib) the door closes a third of the way (2/3 the second time and completely the third time). So far it's bee door open all the way. She checks to see if I am there (she can see me from her bed if she stretches a bit) but no crying. The hope is that over time she will rely less on us waiting outside (and I do my yoga stretches while I wait :).
Every morning we talk about the good sleeping rules, and how she was a great sleeper. We put colorful shining stickers in her chart and make a big deal out of it. She was so proud the first few times she wanted to show her chart to everyone that came by. When she did wake up in the middle of the night I did not make a big deal out of it, but there was no sticker on that day's slot, and when she asked, I told her that she was not such a good sleeper that day, but we can try tonight and see if she gets a shining star the next day.
As for the baby, I am nursing, but the last meal of the day is a bottle of formula for our four month old. I know doctors say it does not work, but it gives this baby two extra hours of sleep vs. breast milk (it was not the case with my older daughter). I would suggest you try.