At a guess, Mary.
An Estabrooks genealogy at:https://books.google.ca/books?id=gkwGA_qNqCIC&pg=PA242&a...
on page 243 names as his daughters:
Mary b. 8SEP1781
Hannah b. 15AUG1783
'Phebe' b. 17JUN1794
The exact birth-dates suggest an access to a family Bible therefore the children listed may well be complete. In Hale's probates at:http://archives.gnb.ca/Search/MC3706/Details.aspx?culture=en...
Hannah Brooks and Phebe Mills are mention in Elijah's will of 1825; therefore both living. The inference is that it was Mary who died in 1812. The problem is that, if a family Bible existed, why was her death not also recorded?
On the third hand, a note at:http://www.ancestraltrails.ca/walker%20for%20web-o/p419.htm#...
(possibly from the above mentioned genealogy?) states that, "Mary died at very young age."