My DS is 7. I tried toilet training him at 2. (Does anyone still call it potty training?) I tried at 3. At 4 he was mostly dry during the day, but still in pull-ups at night. At 7 we still occasionally get accidents. (For the record, DS was born preterm, with IUGR. There have been several physical delays. (No cognitive ones, though) He always catches up eventually. )
If my MIL had said I was being lazy, there would have been fireworks for sure.
Make very sure you know what's going on before you start blaming people. Maybe there is an issue she has chosen not to share with you. Even if there isn't, it still isn't your business. It's definitely considered normal these days to wait until a child is 3 before toilet training.