I LOVE Sarah P.'s answer. Just smile and keep saying, "This is what our doctor recommended." That is, if you see them anymore.
His family sounds like my husband's family - not to be rude, but they are basically uneducated and ignorant. Yet, my MIL thought she knew everything about babies. We had a preemie and she wouldn't even listen to the nurses in the NICU. I joke sometimes that she caused my son to need to stay in the NICU an extra 4 days because she kept rocking him and thumping his back so hard, when the nurses repeatedly asked her not to! His heart got knocked out of rhythm and when he finally came home, he had to be on a heart monitor for 2 months. Maybe she didn't actually cause it, but I just couldn't believe how she couldn't respect that he only weighed 4 pounds and she should listen to the experts. This is just one example. So we just limit contact with that side of the family, except for a couple of family members who are more on the same page with us. It comes into play in so many ways, from the types of toys they buy, to the types of teasing and games they play. My MIL taught my son when he was 2, that it is funny to steal people's chairs when they get up to get the remote, answer the door, or go to the kitchen! It is not funny, it is disrespectful and annoying. Anyway...
You don't need them undermining your good choices and research. It's understandable that you don't want your daughter to suffer for them not liking you - we struggle with the same thing. But she won't suffer right now from not seeing them, and I'd be willing to bet they will be as eager to see her in a year or 18 months, as they are now. Then, when she is actually old enough to enjoy the attention and love they are offering, you might want to have small amounts of contact with them, if you want her to benefit from extended family.
It has helped my husband and I (you could probably do this yourself, or talk to your family about it) to set some limits, at least in our own minds, on how much and what kind of contact we will allow his family, and why. They are not allowed to keep our son overnight, they are not allowed to drive a car with him in it (they have no respect for car seats or seat belt laws), and they aren't allowed to come over unannounced. We do allow my MIL to watch our son sometimes for a few hours, at our house (my brother-in-law's house where she usually stays while visiting is unsafe for children, in my opinion). This allows them some time together without getting on my nerves too much. It also allows her some time where she doesn't feel watched over. But we didn't start this until my son was maybe 18 months old and we were willing to loosen our standards for just an afternoon. At least when they are older, an afternoon of breaking the rules isn't going to hurt them. A little bit of pop, or a bite of food you don't think she is old enough for yet, won't do lasting damage once she is AT LEAST a year old! And as she grows, you can adjust things to suit yourself and her condition. If she has a sensitive tummy, maybe the dad's family can't babysit her until she is older. Whatever works.
Just know that YOU are the mom, YOU know what's best for her (because you have your head on straight), and after all, it IS your responsibility to take care of her, not anyone else's. And you suffer the consequences if they let her get hurt, or feed her something that makes her sick, etc. So it's totally within your rights to limit them if they are unhealthy for her. I was actually kind of a reluctant, timid mom at first but I found my mother instinct very quickly and noone had better mess with me when it comes to my son's welfare! It is okay, really.
Let me add that contact doesn't have to be in person - once your daughter is older, or even now, you could Skype with her dad's family, or later let her talk / listen to them on the phone. You can color pictures and mail them to the grandparents, send them photos, whatever. Kids love to fold up something and put it in an envelope and send it in the mail. We do this with my son, my MIL gets a kick out of it, and it doesn't require her getting in our face about anything.
Good luck!