Giving the gift of a baby

There is an interesting discussion going on today on my birth board (message board for parents of babies born in the same month/year). A woman posted that her husband's sister and her partner (committed lesbian couple) want to use her husband's sperm and the partner's egg to have a baby. It would be a donor-only situation, they don't want to create a family like Arizona/Callie/Mark on Grey's Anatomy where one partner physically has intercourse with the man and it becomes a father/two mothers situation. Instead of the sister and her partner using an anonymous sperm donor, they want their baby to be genetically related to both of them.

The husband is thrilled to do this for his sister. The wife is very upset. She thinks it's incest, that he's betraying her by having a baby with someone else, and seems furious and heartbroken that he'd even consider it.

Personally, I think it is a wonderful thing for the brother to do for his sister and sister-in-law. Can you imagine a better gift to give your sibling than a family? It's not even close to incest like the wife implies - the partner wants to carry the baby. There are many replies on this discussion, arguing both sides. I wonder, though, if the ones opposing would feel the same if the couple was heterosexual? There are many instances of one sister carrying a baby for another who cannot, sometimes even using her own egg. How is that any different? As long as the legal side is taken care of and all sides are protected, I think the husband should go for it. Hopefully after the initial shock wears off, his wife will see the beauty in the situation and be supportive.

I'm curious, though, on others' opinions. What would you do in this situation?