How are Sophie and Diana related?

I'm pretty sure that all that O'Brian ever says is that they are cousins. I'm wondering if there is any way of knowing whether they are related through Sophie's father or through her mother. Can we know anything more specific than just "cousins"?

I thought it was on the father's side, but I'll have to go do a reread to be sure.

Thanks.

It's possible it's one of those really subtle things, where just knowing the social norms of the time tells you. For instance, I know that if the family name is X, the eldest daughter is usually called just Miss X, but the younger daughters are called by their Christian names: Miss Frances or Miss Frances X.

I'm hoping that there are similar hints that will tell us exactly how Sophie and Diana are related.