Here is the item again:
Jean always nibbles around the pit of a cherry, just as if she were eating
a peach,
because she is vain about her smile, she does not want to chip a
tooth on the hard center.
The comma between peach and because causes a comma splice. At this spot, there are two complete sentences joined with just a comma, which is too wimpy a punctuation mark to do the job. Jean always nibbles around the pit ... begins the first complete sentence. Because she is vain about her smile, she does not want ... starts the next complete sentence.
To fix this problem, you could put a period after peach and capitalize the B for because.