“The truth is, we’re very much alike, so naturally I think she’s perfect.”

[mysticgypsy1]

I think Mary means that she and Matthew are similar, and therefore she would have picked Lavinia as well had she been him.

I don't see this line as Mary being sister-ly, necessarily. In fact, I see it as one that solidifies her affinity with Matthew: they are so alike that they even like the same people.

So of course these two people (twins, soulmates, whatever) should be together!

Sigh...