Speaking as a grandmother...
Ignore your MIL and her comments. Don't let her opinion sway you, her opinions are what I would have got form my grandmother. If you can then breast is best.
My Mum could never feed and she has said it is one thing she wishes she could have done.
This is your child and you and your body know what is best.
Keep it up and your DH will wonder what he was thinking in a few months time.
Next time you are shopping together casually stop in front of the formula and let him see how expensive it is.
Re the sippy cup DD1 went from breast to cup and DD2 had a bottle for a few months then to a cup. So if anything is said just say yes I hope she does go straight to a cup.