Seeking information on the family of Nehemiah Covel (aka Covell, Covil, Covill, et al) and his wife Johanna Brooks.

From the census records, it is estimated they were married circa 1820, probably in Greene Co. NY. Johanna was the daughter of Joseph B. Brooks and his wife Rollins Lawrence Brooks of Lexington, Greene Co.

Johanna and Nehemiah are identified in four censuses between 1830 and 1860:

1830 Lexington, Greene Co. NY
"Nehemiah Covill"
* 1 male and 1 female age 30-40, born 1790-1800 (husband and wife)
* 1 female age 10-15, born 1815-1820 (one daughter)
* 2 females age 5-10, born 1820-1825 (two daughters)
* 1 female under age 5, born 1825-1830 (one daughter)

1840 Middletown, Delaware Co. NY
"Nehemiah Covil"

* 1 male age 40-50, born 1790-1800 (husband)
* 1 female age 30-40, born 1800-1810 (wife)
* 1 female age10-15, born 1825-1830 (one daughter)
* 2 females age 5-10, born 1830-1835 (two daughters)
* 2 males and 2 females under age 5, born 1835-1840 (two sons and two daughters)

1850 Middletown, Delaware Co. NY
"Nehemiah Covel"

* Nehemiah Covel, age 56 (1794) NY
* Joanna Covel, age 46 (1804) NY
* Joanna Covel, age 21 (1829) NY
* Martin Covel, age 12 (1838) NY
* Matilda Covel, age 2 (1848) NY

1860 Neversink Twp., Sullivan Co. NY
"Nehemiah Gavel"

* Nehemiah Gavel, age 61 (1799) NY
* Johanna Gavel, age 53 (1807) NY
* Johanna Gavel, age 26 (1834) NY
* Martin Gavel, age 18 (1842) NY
* Matilda Gavel, age 12 (1848) NY

It would appear that the three oldest daughters departed the family either as the result of death or marriage by the time of the 1850 census when the family lived in Middletown, Delaware Co. NY, and that a son died young before that same census.

Who were the three daughters and one son missing from the 1850 census? What became of them? And what became of the Nehemiah, Joanna and their three children last known to be living at the time of the 1860 census?

Any information would be appreciated!