Oh m2boys, you've really got it rough. I'm sooooo lucky my boy got better before DD came along.
One practical suggestion - can your DS sleep with his dad, and you with the little miss? Or other way round - will she settle for her dad (and maybe not reach for the boob so much if you're out of range)?
Certainly, you can't be dealing with them all by yourself. You need a break too - even a few hours here or there makes a big difference. Since I was pregnant with DD, DS was daddy's responsibility, so if he wakes in the night, dad has to deal with it. I have DD. But even then I hand over to DH some nights (she does that 20 minute thing too sometimes!) for a few hours break. And then sometimes I get sleepins on the weekend too.
During the day, can you put a DVD on for the boys and lie down with your DD to nap together?

ETA - *some* babies like to sleep, they just need help. They'll grizzle and settle. Some kids are not like that - no routine, consistency or special method will change that unfortunately, only time. I really hope it's not too much longer for you m2boys.
Just as an aside, have you considered food sensitivities or anything like that?
Master 2 definetely has some food intolerances MadB. But as they seem to be super weird ones I am having a hard time narrowing them down. We are getting there slowly, super slowly. His sleeping has improved with the removal of some foods but not enough for me to be jumping up and down in joy yet. Because we live so rurally I am having a hard time getting help with his tummy issues but we have a paed's appt in the city in 2 weeks because he has a heart murmur so I am going to see if I can get into a naturopath then. DD I haven't noticed any food issues really, ds2's were so noticable. I think with dd I just got in the habit of feeding her to sleep at every noise and so now she doesn't really know how to settle without it. Ds1 had a lot of sleep apnea, his sleep improved a lot once he got his tonsils out, we finally made some headway there. He does sleep a lot better now, but can have restless nights plagued by night terrors sometimes.


I try and nap with dd as ds2 still has a day sleep and ds1 watches a movie. But dd isn't very co-operative. It is a massive battle to try and get her to sleep during the day. So far after the horrid night last night and then being up at 6am she has only slept for 40 minutes the entire day. I tried to lie down with her earlier and she just would not go to sleep. DH won't help at night time so I luck out there lol.