People have always sometimes adopted the children of near relatives who couldn't care for them, at much higher rates than adopting strangers; that's fitness increasing. It's not a stretch to imagine that crossing over to sperm and egg donations.

Both friends and family have the large advantage (at least I imagine it to be so) that they are known quantities: you know them well and can evaluate them as biological parents just as you would a potential mate. I wonder why sperm banks don't seem to offer the sperm of people with known qualities at higher prices: not just screened for diseases, but known to have strong positive traits like good looks and intelligence.