<human>: Could you distill the essence of the text into 1-3 sentences?

To start out, I have an almost 3 year old little boy. And my fiancé and I have been together almost 5 years. We've always wanted children, and we want more in the future. My sister has polycystic ovary disorder, and they aren't sure she can have children. She's single, but wants a child more than anything. She's trying to get pregnant right now via donors. But the outlook isn't looking good for it. I have offered to be a surrogate or gestational carrier for her if she can't get pregnant. I would do anything to help her. My pregnancy was a breeze, I loved being pregnant, and I know the baby wouldn't be mine anyway. But my fiancé doesn't see it that way. He sees it that we would be giving up a baby. I do want more children, but we're in no position to have another right now.  He's made it clear that he's not happy about my choice, but will support me "if he has to". How do I help him understand that this isn't about us, that its about helping my sister be a mom like she's always wanted?
<bot>: I want to help my sister have a baby by carrying it for her, but my fiancé doesn't like the idea. How do I help him come around??