WHAT a FASCINATING story! Some of which can be proved by Census records and other parts might be proven true by other records.
The challenge will be that before Oklahoma became a state in 1907, the entire county of Muskogee was owned by the Cherokee and Creek Nations. Early records were few and sparse. There is a great article Muskogee County, Oklahoma History, www.okgenweb.org/~okmuskog/history 1.html which explains what happened when and where the early records are located.
Charles and Lizzie Morris were indeed brother and sister.
1860 Valparaiso, Porter, Indiana
John H. Morris, b. PA, 40, carpenter
Rebecca, wife, b. PA, 31
Joseph D., b. PA, 12
James R., b. PA, 9
Reuben B., b. PA, 7
Charpes P., b. PA, 4
Mary E., b. IN, 10/12ths.
The family was living there in 1870 too with no additional children and Mary E. is listed as Lizzie.
I'd investigate further Jennie McNulty's marriage to Chas Morris.
Born: abt. 1856, Chicago, Cook, IL
Married: 31 Dec 1879, Chicago, Cook, IL
Born: Chicago, Cook, Illinois
Source: Cook County Illinois Marriage Indexes 1871-1920, Family History Library film #1030111. I don't know if this film will have more info or not but it needs to be looked at or order a copy of the marriage certificate.
I couldn't find any record for Charles and Jennie after their marriage.
I searched through the few cemeteries that have on-line indexes but didn't find her.
Here's a page that provides info about the old newspapers, www.okgenweb.org/~okmuskog/newspapers/newspapers.htm.
You also might want to contact the Muskogee County Genealogical Society, www.okgenweb.org/~okmuskog/geosocieties.htm.
Here's the Muscogee Nation of Oklahoma, www.genealogynation.com/creek/. I didn't look at this but they might have old records.
This web site explains what information is available at the County Courthouse, www.okgenweb.org/~okmuskog/courthouseinfo.htm.
You've got a lot of info to wade through but I think the answers are somewhere; finding them will be the challenge.
1900 Muscogee Nation, Indian Territory
George Bixby, b. MN Sept 1858, clerk (gov't)
Mary E., wife, b. IN Aug 1859
Bradford, b. IN Aug 1893
Oster A., dau, b. MN March 1895
Helen, b. MN Nov 1896
Esther, b. MN July 1899
Floyd Hines, stepson b. IN July 1882, clerk (gov't)
Jennie Morris, neice b. IL April 1883
Irving Smith, day laborer
Voila! Walla! There was jennie Morris. I don't know if she was officially adopted or not.
1910 Franklin, DeKalb, Illinois
Joseph D. Morris, b. PA, 63, marr 34 yrs, keeper hotel
Nellie, wife, b. NY, 52, assistant hotel keeper
Joseph L., b. IL, 30, law office clerk
Lulu Morris, neice, b. OK, 18, no occupation listed
Charles Myers, 21, clerk
Frank Myers, 19, clerk
The rest of the people listed work in the hotel or were boarders.
I also found the Carwile Family Tree, ancestry.com which has the following:
Virginia Rebecca Morris
B. abt. 1883, Illinois
Married: Leroy McMullin Harrington 1 June 1906, Denmark, Washington, Minnesota
They lived in Minnesota.
Most larger libraries provide free access to Ancestry.com.