There's something decidedly wonky here, and I'm not sure that Pleasant and Joseph weren't two different men (brothers? cousins? unrelated?).
Pleasant has the 1865 gravestone, and is cited as the (first?) husband of Mary Lee Lee (she's a double Lee because her father was Uzal Lee Uzal L. Lee ). 1860 Census lists Pleasant, Mary, and son Jonathan W. in Henry County, Iowa.
None of them are found in the 1870 Census for Iowa, though.
Joseph Lee is found in the 1880 census with a wife named Mary, the first three children (including Jonathan W.) - and four more. None of them are noted as "stepchildren".
Levirate and sororate marriage were not that uncommon in small communities - not always on the frontier, either! (I ought to know - my g'g'gfather married two sisters, the oldest and youngest of a large family, and he lived in Maryland.)