To answer your question, my sister in law knew that my husband had to be circumcised at the age of about ten because of a tight foreskin, so she was at least aware of circumcision in that sense. But she still thought of circumcision as something that only Jews and Muslims do, unless also for medical reasons. She didn’t think it was a good idea just to get it done as a routine.
But anyway, she was aware that I got my oldest son circumcised as a baby and was curious then, so I explained my thoughts at the time. Then, after my second son was born, she actually accompanied me to his circumcision at a nearby GP surgery. When she saw how simple and easy it was to get done, and how quickly and beautifully it healed afterwards, she then went to the same clinic to get her son circumcised a year later. It seems to have worked out very well for their family and now she is a supporter of circumcision.