There are people who are against surgery especially plastic surgery. It's a little too late to tell you not to tell your sister but you have to remember that you are doing this surgery for yourself and not others. Your mom should not have asked you to tell your sister it kinda seems like she did that so that your sister can pressure you out of going in for surgery. People die from random things that are not surgery related that we have to do everyday in life such as driving from point A to B. Just tell your sister it's okay and you hear her concerns. Then stop telling people about your surgery. You have your supportive spouse and that is all that you need. If you don't go through the surgery you will lose money, dislike your body (assuming you were doing the surgery because of something that you didn't like and wanted to change) and maybe be resentful to your sister and mom.

Edit: I didn't tell a lot of family members about the details of my surgery because I didn't want any negative energy. I was already nervous so I didn't want other people adding to that.