Hi there,

just wondering - was it recently when her sister started acting like that? If that is the case, I think she might be jealous that you and your fiancée have found eachother and are about to get married. She might be perceiving your steady relationship as a personal loss:
1. Her sister found somebody to start a family with, meaning that she has to move from the center of her sisters life into the periphery. She might feel that she will become less important.
2. She will be living alone now, which can be scary. Her life will be different.

You have to talk to your fiancée about these things. Explain her your concerns. Starting a family is not easy, you will have things to disagree about just for the fact that you grew up in different places and you were taught differently.

I am saying all of this because I went through a similar situation: my mother didn't take it well when I started dating in my mid-twenties. She would try to affect our relationship by saying things like "he will use you and dump you" and "this is just temporary, you will soon find someone else". That was very hurtful at that time because our feeling towards eachother were real. It got a little better when we got engaged and now that we're married she's not talking like that anymore.

Another possible option is that your soon-to-be sister in law is just plain mean. In that case you might have to outsmart her.