Barbara, there are no birth dates shown for the two names. It's possible that Elizabeth may be the sister of Mary Ann. In the visitation records I mentioned there was a Mills family visited on Jan 16, 1775 consisting of parents, John and Margaret with children, Elizabeth and Mary Ann. Names are the only information provided in these particular records and this is the only mention of a Margaret Mills in the entire book.
Given the date of visitation and Mary's birth year, there is only one John Mills in cemetery records who could possibly be her father and he is buried next to her. The noted inscription reads, "John Mills, Died Nov. 9, 1815, aged 83 years." Hope this helps.