My advice to you would be to stop caring about what others think in regards to your daughter, her diagnosis, and the way you go about taking care of her and following up on her needs. People who have never had a child with this condition do not know nor do they understand all that is involved and in some cases, you could explain until your blue in the face and they still wouldn't understand. Everyone is going to offer their advice and opinions to you for what to do - you just kindly accept their help but then do what you feel is right for your child.
I too was very worried about telling people in the family of my son's HF autism. I also felt that they would be "different" around him and treat him differently than they treat the other kids in the family (like cousins at big family get togethers). I just simply told those people who are more involved with him at first - like grandparents, and let them digest it for a while. I highly suggest you get some books on PDD and share information with your father so that he understands more about your daughters condition. Take him to scheduled appointments with you to meet her teachers or therapists or doctors - for instance, your child's next meeting with the doctor that diagnosed her. Or eventually to IEP meetings for school. Get your parents more involved with the very things that help her with her learning disability. Sometimes people don't really believe whats going on because they haven't been there to talk to the doctors and view your daughter during the tests and so on.
Your child has already been pinpointed by family members as being a little "different" in her own way - trust me on this...people who have watched her and seen her behaviors have probably wondered why she does certain things or acts a certain way, so it's not going to be any major shock to some to hear she's PDD. Give things some time and eventually you will become more comfortable discussing things about her with other members of the family, Also things will trickle down as your parents will eventually discuss her with other family members while not in your ear shot. Expect it. Trust that your expectations of her being treated differently will eventually disappear. You will come to see that your fears are unfounded and everyone will love your little girl regardless of the situation. Good luck to you - your not alone in this - there are LOTS of parents out there in your situation!!