The following Lagrange County, Indiana land records suggest that Reba Cass died sometime in the first half of 1838:
On October 10, 1837, David Cass bought the Northwest quarter of the Southeast quarter of Section 9, Township 37 North, Range 8 East, being 40 acres, from Esick Green. Both parties were said to be residents of Lagrange County. [Vol. 3, p. 590]
On June 19, 1837, David Cass bought the NE/4 of the SW/4 of S5 T37N, R8E, being 40 acres, from Hiram Wilkinson and his wife Lydia. Both parties were said to be residents of Lagrange Co. [Vol. 4, p.14]
On January 20, 1838, David Cass and his wife Reba sold their 40 acres in S5 to Nathaniel Glazier. Both parties were said to be residents of Lagrange Co. Both David and Reba Cass signed with their marks. [Vol. 3, p. 591]
On June 24, 1838, David Cass sold the 40 acres in S9 to William Wickham. Both parties were said to be residents of Lagrange Co. There is no mention in this deed of David Cass having a wife to surrender her dower rights in the land. [Vol. 5, p.223]
Both parcels lie in the northwest part of Newbury Twp. On the west, Newbury Twp, Lagrange Co borders on Middlebury Twp of Elkhart Co, IN .

Reba may have been related to Stephen Wickham (1776/7 NY-Oct 1849 IN):
On 14 Dec 1836, Stephen Wickham, a resident of Lagrange Co, IN, purchased 40 acres at the Ft. Wayne, IN land office: the NW/4 of the NE/4 of S36 T38 N in R7E [www.glorecords.blm.gov]. S36 is in the southeast corner of York Twp, Elkhart Co, IN, and borders on Middlebury Twp to the south and on Lagrange County to the east.
The 1840 census of York Twp, Elkhart Co, IN, lists Sephen[sic] Wickham, aged 60 to 70, with three other people in his household.
The 1850 IN, Elkhart Co, Mortality Schedule lists Stephen Wickham, farmer, 72, b in NY, married, d Oct 1849.
The 1850 census of York Twp, Elkhart Co, IN lists Welthy[sic] Wickham, 65, b MA; and three young Wickhams: Hiram 27, Nancy 23, and Stephen 15, all b in Canada.
The will of Stephen Wickham written in Jan 1849 gave his land to his son Hiram.
The 1906 death record of Nancy (Wickham) Mack says that she [like John Cass, son of David and Reba (Wickham) Cass] was born in the late 1820s [in or near] London, Ontario, Canada, and that her parents were Stephen Wickham b NY and Wealthy Munger b Wales [a town in MA]. (Since Hiram was older than Nancy and her parents' marriage record has not been found, it is not known if Wealthy was the mother of Hiram. Hiram died before death records started to be kept in IN.)
Other records have proven that the fifteen year old Stephen Wickham in the referenced 1850 census record was the son of Wilbur and Delila Wickham who married in Ontario, Canada in 1834.
It may just be coincidental that Hiram's daughter was named Reby.