A 6 month old should sleep through the night without needing a feeding. You probably do have to do a little bit of crying it out.
Does she eat baby food? I give my 7 month old a "dinner" the same time the rest of us eat around 6 pm. Babyfood and some sips of water. Then he gets an 8oz bottle after bath (around 7:30), and off to bed for the night. You don't say if your baby's dinner is food or formula/breastmilk. Maybe try doing food at the same time your older one eats, then the nite-nite bottle (or breastfeed) right before you put her down. Then you will know she is good and full for the night.
She may still wake up because of habit, so just leave her to soothe herself back to sleep (does she have a cuddly blanket or anything that helps) OR go in and pat her back, or pick her up for a few minutes and sing a song, then put her back. It may be a lot easier than you are anticipating and she'll fall right back to sleep. Or not, and you'll have to power through a few times of crying.
Remember that babies are always evolving so you will probably not be able to settle into a routine for very long. I also have two older ones, and with the age difference (a 4 and 7 y/o) I cannot do that many bedtime routines without a little help. My husband has to get the older kids moving through bath and PJ time until I can step back in with them. Do you have any help? Is your 5 y/o interested in bathing with the baby? My older kids love to take a "baby bath" and get in with our little guy. The more things you can combine the easier it will be (starting with dinner- have your baby sit in the high chair with all of you at dinner and eat some baby food, whether she is hungry or not, she can sit there and go through the motions).
Another option is to rotate who takes a bath, every other day for the 5 y/o and the baby on the other days.