one simple solution would be to consider getting a double sized bed for the two girls to sleep in together. Put it in a corner if you can..and find some guard rails for the sides. If the younger one really is a snuggler then this may do the trick. My three younger kids all slept with older siblings and have always slept better that way.
Another thing to try is to put the younger one to bed 1/2 an hour to an hour earlier than the older one. When she is sound a sleep you can tuck the other one in quiety.
Routine helps too. Do the exact same thing every night. Bath, snack (a bowl of oatmeal is good!),story time, prayers or whatever is part of your routine, try lavender bath and lotion or massage oil in calming scents on the baby's feet. White noise in the room - humidifier, fan, one of those things that makes a waterfall sound. See if there is a special stuffed animal or blanket she likes and put it with her. Do everything in the same order, every night.. this will help her get used to knowing that this is the bed time routine.
now if she cries.... it won't hurt her. Go in there and DON'T pick her up. Get her to lay down again and rub her back just a little, then walk out and set the timer for 5 minutes. Let her cry. Go back in 5 minutes if she is still crying and do the same thing, then set the timer for 10 minutes, then 20 minutes up to half an hour. She really ought to fall asleep during one of these times. If she doesn't - after the 1/2 an hour try a mini-routine, change her diaper, get her a little drink, read a short story, then put her back to bed and do the whole thing all over again. You may need to have the older child sleep somewhere else until the baby gets the hang of it (explain it to her). This may take 3 or even 4 days but usually a kid will get the message after 2-3 days and the number of times you have to go in will shorten. There will be set backs but treat each one the same way.
You may want to consider how long and how late in the afternoon her last nap of the day is. If it is less than 4 hours before bedtime, consider shortening her nap by getting her up and taking her out for a walk, or do some interesting activity. Avoid running errands when she would fall asleep in the car etc. Do what you can to keep her awake so she can be ready for bedtime when it comes.
best wishes.