Therapy will uncover the reason.
When I was young I had a double ureter and peed the bed...I had no control I was told due to it. I also peed without knowing it throughout the day. I had surgery of some kind while I was young and after that no problem. So if it's not a behavior issue I would make sure all medical issues were checked into.
Please have them check for chronic kidney disease if they haven't already. Or have them do it again. I am speaking from personal experience. If it's not that then take her to a counselor for awhile....maybe it will help her.
Good responses from the others. You know, I first need to say that the average age that boys get potty trained in the US is 3.5 years old. iGirls are known to be a bit earlier but not all. One of my sons was after that at around 4. No joke. And this is my younger, non sensory son. And we weren't alone as I had friends who were working on this same thing with their kids of the same age as my son. My son doesn't have psychological problems nor does he have behavioral problems. He's very bright. (And truth be told, this is how potty 'training' went for him . . . I said "honey, please just get potty trained" and he looked at me and said "okay" and was. Like immediately). Once potty trained, my son had a spell of two weeks in which he wet himself at night due to emotional responses of changing into all day school-- first grade. Hard adjustment for him. My older son got daytime potty trained much earlier but had issues at night with wetting for several years. This is not uncommon. 12 percent of kids have night time wetting until the age of 9 to 10. Most had a parent that also had issues at night. Heavy sleepers have a hard time getting the signal to get up. My friend just got her son to stay dry all night and he is 11--- she used the alarm that they make that wakes a child when they begin to wet.
Okay, so none of that is probably relevant though. To me, I almost wonder about some issues related to emotions because you talk about 'their mom' in such a way that it sounds like you feel she was neglectful. Also, it sounds like a split home. This CAN have an effect. Have these kids ever been abused (sexually or physically) or neglected? But it really doesn't have to be that dramatic. As I mentioned, my son responded to emotional stress of change in school schedule by wetting himself. He did it once AT school as well which was very out of character for him. The books are helpful that you've ordered (or they should be anyway).
Constipation definitely can play a role physically and habit.
What is their feedback on this issue? Kids do become aware of their peers in about second grade (some earlier) and that is usually motivating. You frankly sound very caring-- and if you open up loving and supportive dialogue about this--- and understanding that kids do have to work on things. Some kids are so into what they are doing, that they feel they may need to pee but wait too long and some hits their pants. It may not be a full on pee but 'some' before they get to the bathroom. So, see if you can create a safe environment to talk to them about it.
I think you sound like you are doing a lovely job and hopefully you can get this conquered as I agree, it is in their best interest. All should have a step parent that cares so much rather than just being angry about it. good luck
I'm still curious, and don't feel like I understand - why isn't this a problem at school? If you notice it and smell it, so do her friends - but your thought is that they haven't noticed yet.
Is it not happening at school?
It sounds like the girls have learned how they can stick their thumbs in your eye.
If no one in the school has yet noticed they pee on themselves, it sounds like it's not really a problem at school. Or they pee so little that it can go unnoticed all day.
Have you tried ignoring it? As it is, it sounds like your household routine (food choice limitations, sticker charts, toilet schedules, withholding allowance) revolves completely around the bathroom habits of these two 8 year olds.
If they are doing this at school as they are in your home, it seems it would be impossible for the school to overlook.