Help with rubbery roti/chapatti
If you are using the same dough then the issue lies in your preparation of the dough. There are too many variables to this, I would approach this systematically, eliminating variables as you go by watching your mother in law and measuring how she does things. Specifically I would look for:


Maybe your roti is too thick or too thin, measure the weight of the dough she uses for a single roti and how big she rolls it out, then replicate that yourself
under-cooking or over-cooking, see how hot she makes her pan and how long she cooks the roti
After cooking what do you do with the roti? Does your mother in law seal it in a container and keep the steam in? 


I suspect your roti is too thick, and/or maybe you are cooking it too long at too low a temperature. If it's too thick it needs a longer cooking time and you either have one that's undercooked or one that's rubbery. If your pan is too cool then your roti won't cook quickly enough to trap the air in, letting it deflate and get rubbery. Try rolling them thinner and getting the pan hotter.