You know, I am always amazed at how people feel that they have a right to pass judgment on how others live their lives. I admire those who have taken the time to make an actual decision about whether they want to have children or not. There are so many children out there that are unwanted as a result of carelessness.
I have one child and only had him after thinking long and hard about it. It was the right decision for me and my husband. Surprisingly, we take a lot of grief for having an only child, but we decided early on that we wanted to be able to provide him with a private school education, something we wouldn't be able to afford if we had more. But, this isn't a valid reason for a lot of people; we get the "things always work out" or "your son is going to be lonely without a sibling". He seems just fine to me.
Of course, this is mild compared to what you and others have gone through after sharing your decision to live child-free, but illustrates the fact that no decision is exempt from being judged by others. What comforts me, at times like this, is the understanding that people seem to need to justify their own decisions by putting down those of others. That it is only people who are unhappy with their own circumstances that seem to badger me about mine. My mom used to always say "misery loves company".

Are your sisters-in-law truly happy with their family circumstances or are they perhaps envious that you get to sleep in on Saturdays?
You don't have to explain to anyone why you made the decision to not have children, especially to strangers or casual acquaintances. And, that hygienist was way out of line getting on your case like that. She needs some sensitivity training.
As far as come-backs, it is nice to have something ready to say in response, but I find it easier to just not engage people in debate. If someone is really pushing me on the only kid angle, I just say something along the lines of "you live your life and I'll live mine" and stop the conversation entirely or change the subject. That seems to be more effective and less crazy making for me. So, figure out a couple of statements and practice them with a friend so that you are ready to respond when you need to without anger or frustration. Remember that the only people that have a right to be involved in this decision are you and your husband.
Take care and good luck!