知ってるよ。ついでに言えば、恵子とは「はとこ」だからね。

。、「」。

I know. And while we're at it, Keiko and I are actually second cousins, you know.

Literal

Know [emphatic-よ]. While-at-it say-[if-ば], Keiko [with-とは] 'second-cousin' [so-だからね].

知ってるよ is the casual 'I know.' ついでに言えば ('while we're at it, incidentally') is a discourse marker for adding tangential information. はとこ ('second cousin') is a specific kinship term. だからね asserts the fact with finality. The parenthetical はとこ in quotes suggests the speaker is emphasizing this specific relationship term.