I cannot help you with your search for Jemima Onan, but I can expand your tree further into the present day. I am the 8th generation of your tree.
First of all, I believe that Ray Onan had another son Jason. This is my Great Uncle Jason, and he is still living (just over 90 years old). The 5th generation of the tree would go something like this, but I don't have many of the dates as I haven't done much research:
Generation No. 5
RONLEY WILBUR5 ONAN (RAY E.4, ALEX WILBUR3, GABRIEL2, ALEXANDER) b. Jun 26, 1915 in Lowell, Kent Co, MI, and died Jan 16, 1973, Grand Rapids, Kent Co, MI. He married RUTH PECKHAM.
Children of RONLEY WILBUR ONAN and RUTH PECKHAM are: i.CHERRIE N ONAN ii.DIANNA ONAN iii.GEORGEANN ONAN iv.RICHARD ONAN v.ROBERT ALEC ONAN vi.MERLE LEE ONAN b. 30 Sep, 1936 vii.TERRY ALLEN ONAN b. 1954
Generation No. 6
MERLE LEE6 ONAN (RONLEY WILBUR5, RAY E.4, ALEX WILBUR3, GABRIEL2, ALEXANDER) b. 30 Sep, 1936. He married VIRGINIA ANNE START. She was born on 19 Aug, 1934.
Children of MERLE LEE ONAN and VIRGINIA ANNE START are: i.DONALD JOE ONAN, b. Jun 30, 1957 ii.DOUGLAS JAMES ONAN, b. Sep 10, 1958 iii.DONNA JOICE ONAN, b. Dec 17, 1959
Generation No. 7
DONALD JOE7 ONAN (MERLE LEE6, RONLEY WILBUR5, RAY E.4, ALEX WILBUR3, GABRIEL2, ALEXANDER) was born on Jun 30, 1957. He married ROSEMARY BALLARD, who was born on Apr 11, 1953.
Children of DONALD JOE ONAN and ROSEMARY BALLARD are: i.JEFFREY DOUGLAS ONAN b. Aug 24, 1993 in Lowell, Kent Co, MI. ii.DANIEL JOSEPH ONAN b. Sep 14, 1996 in Lowell, Kent Co, MI.
Well, that's all I've got to add. I don't have anything recorded except for my direct lineage, so all of my cousins, or my dad's cousins, or his dad's cousins are not on this tree. I also have very little here as far as birth dates/locations. Most if not all of the people I've recorded here are still living.