OP, there is good advice here for if you want to express for bottle feeds, leave your DD with your ILs, mix-feed, etc. But you certainly don't have to do any of that.
Breastfeeding is a wonderful thing to do for your baby (if you want to/ can manage it), don't feel pressured to stop or to compromise your supply by giving formula instead. It's nothing at all to do with your ILs, so you have every right to either completely ignore them or firmly tell them to back off. If you want to though, you could educate them about the benefits of breastfeeding to both you and your DD, which might help them understand how selfish they are being.
And maybe look into going to a breastfeeding support group- the health visitors at our GPs surgery run one. Even if you're not physically struggling with breastfeeding, it can help to normalise it to make some friends who are also breastfeeding. It will also give you somewhere to complain about the harder bits, as I imagine if you do that at home it will fuel the argument for stopping.
FWIW, my DD is 14 months, has never had formula, and has only recently started spending some time with my ILs without me. She still has a great relationship with them, and has special little games that she only plays with granny. You certainly aren't cutting anyone out by breastfeeding. Good luck 