Why would it be traumatic to find out you have a lot of half-siblings? I can easily imagine it would be disappointing, or exciting, depending on how you feel about missing out on time with them vs the possibility of future time with them.

I wonder if DNA ancestry testing is popular in Australia. It would be really cool if these places that provide donor services were able/willing to provide recipients or the offspring with free test kits to help avoid unpleasant surprises.