This is how it happened with me and and my husband. I told him I was doing it, and asked for his help in encouraging me to stay on plan. What resonated with him was I said I had no idea how long I would live, but I intended to spend that time as a thin person, able to do things I wanted to before I died. He realized he wanted the same thing.
I can NOT tell you how beneficial it has been to have a buddy go through it at the same time. We did our pre-op together, we had surgery the same day, and now we're recovering together. We can support each other, we can share experiences, and we can take care of each other.
I don't know if your sister lives close by, but it would be so good for both of you to be able to help each other through this, even if she lives far away.