I think this is a difficult dilemma. My immediate instinct is that blocking illegal material is obviously an invasion of privacy. It is impossible to block one type of message without first reading all messages and classifying them.
But on the other hand, we're talking about other people's servers here. They shouldn't have to host illegal material. In fact, it is illegal for them to do so. So it is their right to know what they're hosting and clean it out.
Should we really have any expectation of privacy on big tech platforms? If you're sending obviously illegal material in plain bytes to a Microsoft server, what do you think is going to happen?

It's step, not half-siblings. The man in question is only related through civil contracts between your parent and his parent and, keeping with the lore of the step video, those civil contracts were only created after both of you reached the legal age of consent. The taboo is merely in the legal ambiguity of the situation.