I found next to nothing about all of them.
1860 DeWitt Co. TX
J. B. Alexander, b. TN, 36, farmer
Sarah C., b. MS, 25
Bradford, b. MS, 8
Albert, b. TX, 6
James, b. TX, 4
Samuel, b. TX, 11/12ths
I don't think Tennessee was recording births when J. B. was born. I know they weren't recording them that early in Texas.
Look for J. B. and Sarah's marriage document. Both sets of parents might be on it. If not, then look for their marriage announcement.
I didn't find them in an 1870 Census.
1880 Karnes Co. TX
William H. Drake & family
Sallie Alexander, b. MS, 47, widow, boarder
Richard C., b. TX, 8, boarder
I didn't find any additional information.
Karnes County records are on microfilm at the Texas State Library and Archives, https://www.tsl.texas.gov/arc/local/karnes.html
. Your local, public library can request the microfilm. They have probate records starting in 1865-1907.
Deaths were first recorded in Karnes County in 1903.
I'd contact the Helena, Teas and Karnes County Historical Society, https://helenatx.com/
and ask if they have a local death index, indexed newspaper death notices and cemetery inventories. John's parents names might be in his death notice. Successful Searching!