Hi J.,
-You mention that this has been going on for "the past couple of weeks". Could it be since school started for the older two?
-I recommend building your baby's weekday schedule based on the older girls' schedule. Don't try to maintain the schedule on weekends. You may as well not go overboard trying to put her down for a nap when they are home, because of the noise and distraction.
-Like the other postings, I strongly recommend some sort of white noise. I used the "awake, feed, play" cycle for mine.
-So, try waking her up early with the girls. Feed her. Then exercise her; play with her, do calisthenics with her, get her outside, go to a playground, whatever. Then, take her home (without letting her fall asleep in the car), feed her a snack, take her to her room and make her comfortable and read to her. If she doesn't doze off for the first few days, she will after she gets used to the pattern of this being "quiet time".
-Repeat the same cycle in the afternoon. Don't be surprised if you have to put her to sleep at the same time as the older girls in the evening.
Good luck!