Are Jace and Clary really brother and sister ( Mortal Instruments )?

So I just finished reading City of Bones and I found out something that I was kinda grossed out by and found extremely disturbing. That Jace and Clary were brother and sister! They kissed for goodness sake and now there brother and sister , I just can’t stand that, what I’m wondering is if that’s true or if valentine was just saying that??

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