I've had both experiences. With my sister, it was easier. I prefaced it with, "Feel free to tell me to STHU, but ..." I just told her what I knew, and offered to send her more information and links. I did, and she decided not to circ her first child, a son.
I've had two other times where it didn't feel right to push the issue. Once was at a party with moms and dads and kids, and the issue of circ came up sort of non-chalantly, in passing, and I realized everyone in the discussion had circumcised. I said nothing during the discussion, because there was circ fervor in the group. There was one pregnant woman there with her first child, a boy, so I found her later by herself and told her that circumcision really isn't necessary. She said that they had decided they were going to do it "for the health benefits." We are not good friends, and she did not leave any room for further discussion, so I dropped it.
The second time was with my nanny. She is also pregnant with her first child, a boy, and she actually asked me about it first. Her question seemed a bit searching, like she hoped I would say that I do/will circumcise, and that would make it ok for her too. I told her I would never circumcise a baby, that my DH is intact, and that even if he wasn't, I would never do that to my child. Then, since she had opened the subject, I asked her, and she said they were going to do it as "part of their religion." She attends a large non-denominational christian church, but anyways. I left it at that, because she had invoked the bullet-proof religious clause, and also because technically she is my employee. I feel she is misguided, but I don't want to push the issue, especially when she is still caring for my daughter. I did order the "What your doctor may not tell you about circumcision" book, and I plan to leave it lying around at my house when it arrives.
It's hard. I am not a very social person (maybe high-functioning ASD?), and it's hard for me to guage other people's reactions sometimes. I have been burned too many times with non-circ related issues where I just didn't get it, and stepped over some boundary that I didn't see beforehand. I feel like I do have a very heightened sense of empathy though, and I just don't want to do any more harm than good for the cause.