I have 10 mo old b/g twins and also had issues with sleeping. Do you have a portable crib? You can set it up in another area and have her use it until you break this cycle. Then, when she's back on track, move her into her regular crib near her brother.
We also used water. Start with the largest amount she typically drinks. Then, each night, reduce it by 1 oz until it's not offered or needed anymore.
Also, try to watch how much she's sleeping during the day. I read Ferber's book and he suggested total nap times- I think 5 mo. was about 2 1/2 hours. But that includes all sleeping during the day- naps- in the car, etc. When I started adding up all the min. they slept, it was way over the amount. Perhaps cutting her back a little during the day will help at night.
Don't get discouraged. Like I said, we had issues too. Then we did the Ferber method and everything was going really well for over a month. My son got sick and now we've had issues with him waking too early too (5 instead of 6:30 / 7:00). I'm going to try monitoring the daytime sleep, putting him to bed a bit later and then using the portable crib to have him in another room so we can "Ferber" again w/o waking his sister. Best of luck!