Mrs Harville must be an odd mother to part with them so long. I do not understand it.

How ironic for Mary to say that. What about Mary herself, when she left her son after he had that fall (ch 2)! and -

I can leave them at the Great House very well, for a month or six weeks.

So, Mary can leave her children when she wants to, but if anyone else does it, she's an 'odd mother'?

Such a complaining letter! I think the only part I can agree with her is:

What an immense time Mrs Clay has been staying with Elizabeth!