To potty train or let her pee and poo in diapers?? That is my question.

I have 2 SDs aged 4 & 2 1/2. I have been in these kids lives since OSDs 2yr bday. And YSD was 3mo. I am a SAHM of 1 (11mo on the 9th) so I have all 3 kids all day while SO works. Its ok because we cannot afford daycare and I am not ready to let BD go to daycare until she is able to communicate should anything happen. We gwt skids for 1-2 mo at a time 2-3 times a yr. The CO states we get them 1 WE every month, every christian holiday (BM is wiccan and she gets all wiccan holidays), and all time based on SO's leave from the military. Of course all of this has to be aggreed upon by BM.

Anywho... I do all of the teaching and harder parenting of the skids. I had to potty train OSD at 2 1/2 because BM was too lazy. She didn't even know what a potty was when we started. When she learned she did great. Caught on quick, super excited about wearing big girl panties, very few accidents etc. She goes back to BM and all the sudden starts having regular accidents and BM gets p*ssed because #1 she keeps having accidents and #2 I had no right to teach her kid to use the potty because I'm not her mom. What-the-f*ck-ever. Fast-forward to this visit... we have had skids at home for 4 days now after a 4 day camping trip. OSD had no accidents while camping despite a short walk to the facilities. In the first 3 days we have been home she has peed on my couch (when I asked her why she said she thought it was a good idea) and in her bed 2x. I am so frustrated...

YSD has barely begun potty training @ 2 1/2!!! My dilemna is do I work on potty training her so I have fewer diapers to change and because she is starting headstart this year?? Or do I let her keep peeing and pooping in a diaper like a baby?? I feel like its not my responsibility to keep teaching these kids everything when we don't get them much anyways, however it means I have a ton of diapers to change daily. Do I risk pissing off BM and get this kid trained or do I deal with it?? I'm hoping I can get some opinions... either way its more work for me. I just wish this lazy b*tch would take care of her damn kids and not expect me to do everything!!!

On another note... she has had both skids tested for autism because they have consistently fallen behind on their milestones. Now YSD is in speach therapy. Once again her 3rd child with her current SO is also way behind on his milestones. He is 2 days older than my BD but about 3-4 mo behind her developmentally. Is it really possible that all of her kids have mental defects (according to her... I see nothing wrong with any of them) or could it be that she is just too lazy to actually work with them?? I don't get it...