I agree that if they had mentioned Winthrop as their intended destination, Mary certainly would have declined to come--and since they really didn't want her along, it's surprising that they did exactly the reverse of what they wanted. After four years, they ought to be familiar enough with the quirks and perverseness of Mary's personality to have realized what would likely happen.

I guess a better question is why didn't they really want anyone to know they were going to Winthrop? Charles (their brother) would have been pleased, probably. It doesn't seem so very odd that they would walk over to visit their aunt and cousins. Why the big secret?

And, as I said I feel sure that Louisa was the engineer of this whole plan, but I wonder how much Henrietta had to say about it. It seems like she knew, but then again it also seems like she was deliberately being cold to Charles Hayter.