My first son was 20 months and the second 2 yrs 5 months. With son no.1, we went on holiday and our apartment had a large veranda with a grid in the middle of it. He was running around naked and we noticed that when he wanted a wee, he would do it in the grid! So he had the control and the ability...when we got home, we did the same with the potty. It was plain sailing. Had we not had been on that holiday, I would never have even considered that he was ready to use the toilet himself!
Second son - At about 2yrs 4 months, the weather was warm, so I repeated the above, with a potty in the garden, and eventually he started doing the same as his big bro had with the grid!!
What I am saying, in a long winded way is they are ready when they are ready and that is the practical reality. Yourmother should back off - there's enough pressure in bringing up little ones without this type of unhelpful manipulation!!