I can't give advice as Im not a doctor, but I had Cin 3 as well. I did have them removed before I was pregnant but I was meant to have my check up and I got pregnant, so I didn't go for the check. I was told however, that it would be perfectly safe and wouldn't harm the baby in any way. If it's ok to take the cells whilst pregnant, I can't see why they couldn't remove them as well. Although the op did involve general anesthetic, but maybe they can do it another way. It would be worth talking with teh Dr about all options, but if it came down to it, i think I would wait to have the op until after teh baby is born. Im not sure, but i think my Doc told me they wouldn't turn in to cancer straight away, and Im sure it does take a while. I have just had my check recently and there's no sign of any abnormal cells whatsoever, and I think my cells were pretty bad at the time.
Good luck for your sister and I hope she finds an easy way to deal with this xoxo