Right. This is ridiculous. I don't want to come across as a "breast-feeding Nazi" (see the story at the end of this post for my rebuttal), but it is good for both of you. It is the best for both of you, although as I say below, I hate the guilt that some mothers experience when they can't bf; such guilt is so useless!
If you want to be very balanced about it, look at it this way. Being a mother is performing a complex role. Feeding is one part of it, a very important part, although not at the expense of health and mental well-being. That is: if a mother goes mad because of pain of bf, or has no sleep, because she is trying for hours to feed, she would be a better mother by gaining back time and comfort by bottle feeding. So here your family are undermining not only your feeding your child, but also undermining your judgement, your wishes, you, and they're doing it at a time when you are run down and need support, not nagging. Thanks a bloody lot! (for the record, I am not bitter because of any personal history. I was totally ff, but my mother never tried to make me do the same).
And don't forget that formula feeding isn't exactly easy, either! There are endless bottles to wash and sterilise, and the all the faff of getting the formula powder in (and not all over the floor and in your hair), getting the bottle to the right temperature, making up another bottle when DC is having a growth spurt.... If you can breastfeed, once the feeding is established, it's so much easier: no bottles, no sterilising, feed to sleep, feed while asleep, always at the right temperature, little wastage (unless DC unexpectedly sleeps through, in which case you might have to express!). But you get the picture.
Again, if you can't bf, you mustn't feel guilty about it. I have a friend who carried on through endless infections, a tongue tie diagnosed late, and so on, and I was a bit stunned at the level of support she received from her family. In fact, I think she took it far too far, and was just not catching enough sleep and good health to just enjoy her baby and bond. But that's a different matter!