Yes I think our mum's generation were made to believe that FM is superior to breast milk, probably because of the way it was marketed back then. When I had my first child, my mum was quite sure that my supply was not enough (didn't help that the baby cried a lot) so she insisted on feeding formula so I only partial BF-ed. But when my second one came along, I felt more prepared and determined that I could at least fully BF for the first month, and 5 months later, still happily BFing despite my mum making comments and whining/nagging about giving the baby the bottle during the early days. I was living with her when my first child was born but not with #2, so it was also a lot easier to ignore her requests :P
So, can look on the positive side, your mother in law thinks FM is superior to breast milk so she just wants you to do what's best for the baby. She's just misguided, that's all. Maybe you can tell her that even the Government advice mums to BF for first 6 months (it's printed on all Stage 1 FM tins) or if she follows you to the gynae, tell the doc to advise her on the matter, some of them will only listen to advice from an authority figure.
Ultimately, it's your baby and both you (and your husband, is he supportive?) should send her a clear signal on where you stand on the matter and dont't be cowed by her "threats".