M.S.
We just had almost the exact same situation happen to use. We tried everything, except letting her cry since she shares a room with her sister. We decided to have our older daughter come into our room when she started to cry so her sleep was not interupted. I then went and slept in my daughter's bed, so the baby could see me. She actually went back to bed, without crying. I think she just wanted to know I was there with her. This went on about two more nights, and now she is sleeping through the night again.
We also thought that our older daughter was sleeping through the crying. I had told her before she went to bed if she heard her sister crying to get up and go into Mom and Dads bed. That same night when the baby cried, she immediatly got up and went into my bed. So, we were wrong and the crying did wake her up every night. Good luck!!