I'm sorry it's not an easy thing to watch, and one of the biggest downsides to breeding.
To me it sounds like Mom wasn't frantic to keep it in the pouch, but that might of been the beginning of the rejection, I didn't see it, but that is what it sounds like to me.
She may not have been getting all she needed from the diet. I know many places say to up the protein for moms with joeys in pouch.
It is more recommended to up the portions of everything fruit, veg, staple, not just the protein. This way all the vitamins and minerals are increased across the diet not just the protein.
When feeding well known balanced diets like Bml or Critter Love we typically increase the portion we usually feed, 1 more tablespoon of staple, 1 more tablespoon of fruit and 1 more tablespoon of veg. Only increasing one part of it makes the diet unbalanced and won't provide all the necessary nutrients nursing moms need.
At first Mom eats it or as much as she needs of it, when mom starts weening the joeys the joeys begin to eat that portion and mom eats less of it because she is needing less.
I would suggest putting them on a well known diet and if she rejects the second joey def start thinking about getting the males neutered or separating the female from the intact males.
Most breeders will do one or the other if they reject twice. After twice you continue to let them breed it's bordering on animal abuse, many say it is animal abuse no question about it.
Considering you don't know if they have rejected joeys in there previous owners care I would def neuter or separate and not give them anymore chances, you don't know if this is the first time or one of many that were just never witnessed, and that is a realistic possibility.
It's not easy and I'm not placing the blame on you, you are dealing with the consequences of someone else's decisions and or lack of knowledge about gliders and breeding, it happens a lot. But now they are in your care and you have the opportunity to make more responsible and educated decisions for them.