Many people hear the phrase "cry it out" and immediately think the baby is laying there for hours screaming itself to sleep. Not necessarily the case. I would not advise that method. But most people who use a sleep method called crying it out do a lot of checking on their kids. Its important for kids to learn to self soothe. A pacifier or thumb to suck helps, but some kids want nothing to do with that. Having the room dark and quiet (or using some white noise - like a box fan - to drown out the household noises) helps a lot too. Some kids can sleep anywhere, others need the environment to be perfect.
My kids sleep much better in a quiet dark room, but they have learned (though lots of traveling) to sleep in the car or in a brighter noisier room. However they do cry longer.
If you're interested in crying it out I'd suggest researching the different methods out there. Dr. Ferber has one (http://www.amazon.com/Solve-Your-Childs-Sleep-Problems/dp...) that you can buy or find in your local library. Also you can watch some SuperNanny episodes on www.hulu.com to see how she handles it (slightly different, but still affective). Basically my biggest advise is pick a method and be consistent.
Probably a lot of this is attention seeking, older child is back with you again, has competition from younger baby for your attention, etc... Make sure you're carving out time for the older one as much as possible.
I would NOT advise letting her sleep with you. That will solve NOTHING!!!! It will be a new habit that you have to break later on. I also don't advise laying down with her as she sleeps, again, she's not learning to self soothe but rather to NEED YOU to fall asleep. She is needing to learn that when you put her down to bed, you're there if she needs you, and you're there when she wakes up. Its okay to be alone to sleep. It might take a while, but she can learn this and it will make her a more confident and independent child!
Also 15 min of crying at bedtime is normal and okay for a child. Parents who are AFRAID to let their kids cry end up being super manipulated by the kid. All the child has to do is cry and mom jumps in to do whatever it takes to make them stop. Imagine how these kids are when they are older - teens!
Best wishes! I can't imagine having two babies in the house, my 16 month old is enough, lol! Hang in there!