Well, there's nothing to say she didn't tell Mr Greengrass later, and they kept it from Daphne, but the thing is, with women, it's easier to just pretend that the baby belongs to the man you're marrying. If she knew for sure, then I think she'd had been more like Michael's mum and told her husband, but then they decided not to tell him.

Of course, in that case they were preserving the fiction that he was Pureblood, as you say. So they had a vested interest in pretending he belonged to Mr Corner. And you'll recall he was proper ashamed when he found out!