Look to be fair, I think the OP was looking out for her Dad and the event came as a big shock to er and her family and she doesnt want her Mum upset. It WOULD be a big sock to find out that not only do you have a half sister, but that she is in the public eye as well. Then there is the complicating issue of the fact that it was revealed in a public way so there is no way of possibly 'protecting' her mother from the knowledge, so OP knows that she is about to enter a very hard and upsetting time with wide ramifications for her and her family, not to mention any press that may get involved. Yes, she was a little hasty in revealing who it was (well, no, another poster did from info OP gave!) but she is probably a little upset right now and not thinking straight.
So how about a little thought for what the OP is going through, an upsetting time, and cut her a little slack?