SuperNanny has a technique for this. She says to calmly put the toddler or preschooler back to bed without saying anything. Keep doing this until they know they have to stay in bed. Since this is in the middle of the night I think that giving her a reassuring hug would be helpful. But don't give her a bottle. If she doesn't already have a "lovey" perhaps you could shop with her for a special stuffed animal to take to bed with her. Substitute the "lovey" for the bottle. Do the same with the baby. My daughter had a doll that would light up when you pressed on it.
I've read that this training only takes a few nights if you consistently stick with it. I wouldn't let a child cry hysterically but a little crying can be self soothing and help her go back to sleep. If the crying gets serious and lasts more than 5-10 minutes, I'd go back and reassure her, hand her the lovey, and leave. You could put her pacifier where she can find it herself. Perhaps under her pillow. Tell her you know it's hard to go back to sleep but you know that she can do it. There are some stuffed animals that play music when you push on their ear or paw. Perhaps that would help her.
It is a manner of training or allowing them to learn self soothing ways to get back to sleep. Your daughter has you trained to get up and give her a bottle. I understand that doing this is easier for now but in the long run it is not. It would be good for you and your husband to get a good nights sleep. To have that is worth a few nights with less sleep while you help her learn how to go back to sleep by herself.
This may be difficult for you to do. Is it possible that you are over protecting your sensitive child? Do you try to "make things a certain way" for her? If she cries do you give her what she wants? I wonder if you're overly sensitive to your overly sensitive girl. Babies and children sense when their mother is anxious which makes them more anxious.
Is it possible that the 15 month old wakes her up by snorting or snuffling or whimpering just a bit. Perhaps he's the first to cry. Would it be possible to have him sleep in another room to see if she sleeps better? That's tricky to do and might not be a good idea. It could disrupt his sleep pattern. He also may need help in learning how to self-soothe.
If they don't already have a night light put one in. You could try "white noise" that would cover up the night sounds. My daughter slept best with a fan blowing on her. She still gets too warm during the night and still uses a fan even tho she is an adult.
It's possible that she's having "bad" dreams. To help prevent this be sure that the night time going to bed ritual is soothing. do the same things every night at the same time. Especially read a happy book or tell a nurturing story before turning off the lights. My daughter plays a sleepy time CD after she leaves the room.
My granddaughter had difficulty getting to sleep. I think she was about the age of your daughter. She and I would make up a happy story together. I always put soft floating clouds into the story. She nearly always put a rainbow in. We included flowers, soft cuddly animals, and little girls like her. She was usually in the story too. I ended the story with suggestions that the characters where safe and sleepy. "Now it's time for you to sleep." Hugs were important. Sometimes she wanted to talk about something that had scared her. Sometimems we put that into the story and made it turn out alright. Other times she just wanted to talk about it.
I also gave her a soft cuddly teddy bear. She hadn't been sleeping with any one thing up until then. She still sleeps with that bear part of the time. She's 8. She vomited on the bear and washing it turned it's fur rough looking and not so soft. I bought another one like that one but she rejected it. I found the bear at the Dollar store. It was floppy and easy to snuggle with.
The goal is to help her learn how to self-soothe herself back to sleep so you can get some sleep.