Sister in law and her wife are breaking us

Well I’m getting married in a few months and I wanted to wait a year or two before we start trying for a family. But my fiancé’s sister wants a child with her wife, and her wife is pressuring her to keep asking my fiancé when he can donate sperm. And he really wanted to wait till we have a baby first. But I don’t want any yet like I said. We have been fighting about it for a couple of days because I told him I don’t want him to donate sperm to the wife since she’s very pushy and keeps pressuring him. I also don’t feel comfortable with my soon to be husband having other children other than our own. I know I’m selfish and a horrible person, but it’s just how I am. The wife even blamed my fiancé when they were buying bridesmaid dresses saying that everyone was against them getting married. It’s getting to the point where I’m not even sure if I want to have children anymore or even get married. I don’t know what to do anymore.