The NYT has a fascinating, powerful story of Donor Sibling Registry, the online database for uniting sperm donors with their genetic offspring which has, in the mean time, been busy uniting the half-siblings. The mothers and half-sibs are generally keeping in touch, forming loosely defined family networks around these invisible donors, who are known only by their sperm bank donor number and a brief list of education, hobby, and physical characteristics (which, unsurprisingly, the kids often share.)
Parents are increasingly open with donor insemination (D.I.) kids about their genetic origins, and the kids seem to be driving a lot of the efforts to find other siblings. Then they say things like this: "This is my sister from another mother, and this is my brother from another mother, this is my other sister from another mother..."