Fully understand your situation because I experienced the same for my 1st baby. At that time, my mother-in-law helped me for the 1st month. She always said similar words as your mother, questioning my milk supply, telling other relatives that I don't have enough milk, so need to supplement with formula. Such 'speech' really hurt my a lot, but I can't 'scold' her.
For my second baby now, I gather more information about BF to increase my confidence. And my mother helps me for the 1st month, while I told her beforehand that I decided to fully BF this time, pls support me.
One way may help you from stopping your mother's 'speech' is to bring her to the Health Centre and let her listen to what the doctor or nurse said. I found that my mother believe what professionals said more than me. Because the doctor/nurse said my baby's increases very good & have enough wet diapers; my fully BF is very successful & ask me to continue. It seems that she then show more support to BF. Anyway, she quite support BF beforehand because she also did the same before.
Add oil, don't give up. Try your best to ignore what others' said. Believe yourself & get more info from BK to increase your confidence.