Have you heard about Abraham's wife? No, not that one, everyone has heard of that one. No, not the other one, she wasn't a wife. The third one! (Or was she the second? Or was she not a wife?)
So, yeah, Sarah (that one) died, and Hagar (the other one) was definitely not a wife, but good old Abraham needed some lovin' in his advanced age. (If Sarah had Isaac when Abraham was 99... you know, it's best not to think about it.)
Enter Keturah! And while Abraham only had one son with Sarah, and only one with Hagar, Keturah gave him six! (At the age of... it's best not to think about it!)
So who was this mysterious Keturah? Scholars do not agree. Some believe that she and Hagar were the same person, others do not. Some think she was a wife, others reduce her to concubine status. Some think it's just mythology! (Oh, they are definitely right...)
But this is not WifeOrNotWife.com or ConcubineOrNotConcubine.com, this is JewOrNotJew.com, so we have to figure that part out. Scholars do not consider Abraham's children from Keturah to be Jews, so we would have to deduce that, by the traditional definition, she wasn't Jewish herself.
(It's really creepy, you know. A hundred-year-old dude shtupping what must have been a much younger... Alright, we thought about it!)