Isebel is unknowned in mediaeval Spanish, in my knowledge.
In Middle Ages, there are some variants of
Elisabet (the main form of the name): Elisabel, Elisavel, Elisaven, Elisabe, Elisava, etc. After, in 12th century, the beginning El- was mixed up with the masculine article "el", unsuited for a feminine name: Elisabel>El+Isabel>Isabel.
Note: some people put afore the article in vulgar uses of Spanish (this is a serious mispell, of course): el
Antonio, la Juani, etc.