My sister in law is in her 30s and unmarried. She wants to have her own child and I don't want to be the sperm donor

My sister in law is in her 30s and unmarried. She is very serious about having her own child. She has been pressuring me to donate my sperm for her to conceive. I don't want to be the sperm donor. I don't have money right now to buy a vasectomy so I can't get a vasectomy, so I feel like I am stuck.