You definitely don't need to feed your baby formula. Not unless breastfeeding doesn't work out for you for whatever reason, and obviously your baby isn't born yet so there's no reason to believe that should be the case.
I have had a wonderful breastfeeding journey with my two children and neither of them have ever tasted formula.
What I would say is that if you do manage to exclusively breastfeed, it might not be a bad idea to introduce a bottle occasionally once your supply is established. My son started to take a bottle of expressed breastmilk once or twice a week from when he was about a month old and it meant I was able to send breastmilk to the childminder's for him after I went back to work, until he was big enough not to need it. With my daughter we weren't so deliberate about it, she took a bottle a couple of times when she was small and then didn't bother doing it regularly, and by the time I went back to work when she was 8 months old she wouldn't take a bottle anymore. It has made things more constraining for us because I was the only one who could feed her.
This is not to say that you should do this for your MIL's benefit, but you might be glad your baby can take the occasional bottle if you want to go out for the evening, or even if you were ill and needed to go to hospital, you'd know that your baby would be OK.
But when your baby is newborn, if you want breastfeeding to be a success you should keep your baby as close to you as possible for the first few months, feed on demand until your supply is properly established, and don't let anyone else interfere. Your MIL needs to respect your boundaries.