It sounds like quite a personal choice i think. I did ask my family to get the vaccine before they came and visit just so to protect the baby as their immune system is weak. I dont think you are necessary wrong just different perspectives.

As far as I am aware ( with all vaccines ) , I think if all the children had theirs from the immunisation schedules etc , then automatically we are protected. It may only be risky if particular families don’t vaccine due to religious beliefs or whatever that the environment becomes a health risk . Maybe ask your sister in law what her concerns are as I am the type to ask questions and even if you didn’t mind, would she be willing to pay for it as it will charged if you have as an adult . She maybe a very anxious person who worries a lot especially if it is her first pregnancy . Hope you sort it out soon .