Fenton's long-lost brother

[SDLagent]

Or she's the daughter of one of Janet's siblings. If she was Fenton's brother's kid she wouldn't call Sue her cousin.

[tomswift2002]

On page 174 in Super Sleuths! Volume 1, it seems that Karen is related to Frank and Joe through marriage, and is maybe a "cousin", but is not a "blood cousin", since here is how the passage goes:

Nancy asked,  "Isn't Sue related to Frank and Joe Hardy?

"Yes.  They're her cousins."

By the use of the pronoun "her", Karen is saying that while she is related to Sue, she is not related, by blood anyway, to the Hardy Boys.  Of course, Karen could be meaning that she is a distant cousin (ie. 4th or 5th cousin).

[MacGyver]

Good eye, sir.
Okay- I think that clue of the pronoun definitely sheds light on this- I think the cousin by marriage explanation probably fits best.