(((Hugs))), sweety. It will get better and I can imagine it's very difficult for you right now not spending time with DS.
Have you tried the sling? Would she sleep in that? You could pop her in it at those hours and just do whatever you need to do.
I agree about the AP but I still don't think you should spend 4h trying to get her to sleep or stay asleep. You'll go mad, won't you?
I would have a more clear BT routine. You can, say, put her pj, walk her around the flat and say bye bye to all the rooms, say good night to her big brother, take her to her room, close curtains, feed her in the dark and put her in the crib. Throughout it talk to her with a gentle quiet voice, tell her what you are doing and that it's now her sleepy time. Repeat the exact same thing every night.
It's ok for you to feed her to sleep for this sleep. We can just hope that she will fall asleep on you with this feed and will continue sleeping till her next NF. Somehow it needs to be clear for her that it's BT, that now it's night and time for sleep.
Maybe till now things were a bit confusing for her, she didn't know if it's her A time or S time and probably got OT by not getting a restorative sleep during those hours.
When is your DS BT? Would you be able to put her to sleep after he is in bed that you three can spend this time together? As long as it's not too late for her, of course.