A.J.
sometimes at about that age, babies start waking more because they are going through a growth spurt and need extra formula at night or they are learning so many new physical skills like crawling or sitting that they can't relax during the night. I know you don't want to wake your older daughter, but my kids share a room (they are 4 and 2) and when one of them wakes in the middle of the night, the other is so used to it by now that they don't wake up. I would try just waiting a couple minutes before going in to soothe your daughter. My pediatrician suggested this with my first daughter and I ended up doing it with both my girls and it worked great! The first few nights, wait only a minute or two, then go in and do whatever you would normally do to soothe her. After a few nights, increase the time by just another minute. It sounds like a very short amount of time, but when you are laying there hearing her fuss it will seem like forever =) But just giving her a few minutes to see if she can soothe herself will help her learn that important skill. It only took my daughter 2 weeks of slowly increasing the time every few nights and she was falling back to sleep on her own.
Hope this helps and works for you!