I know this again doesn't answer your initial query, but might be relevant to you in other areas:
While looking to see if Samuel or Joseph left wills, I didn't see them but there are:
Daniel SMALL, Ash next Sandwich, probate 1713 (might be the father of Samuel and Joseph)
Elizabeth SMALL, Ash, probate 1734, widow (wife of Daniel iirc)
http://wills.canterbury-cathedral.org/In terms of using wills, if the potential siblings are each traced, if someone left a will there might be a slight chance of your Mary being named in it (with married name), although often such wills were for immediate family. Sadly, unless the Marys were born in 1733 and christened in 1734, they won't be named on Elizabeth's will as she made it the year before the probate- nevertheless it might be an interesting window into your family at the time.