1.  Virginia Will Records, Berkeley County, W. Va. (online at GenealogyLibrary.com):

p. 537

In the clerk's office of Berkeley County, at Martinsburg, are recorded the wills and deeds of many distinguished persons, who at different times resided in the county. 1.  Will of James Rumsey, Inventor of the Steamboat. Being in sound health, boath of body and mind, I make and ordain this my last will and testament.  Item, it is my will that all my estate, both real and personal, shall be devised as followth, towit:  One third to my affectionate wife and the remainder to be divided into equal shares, two of which to be given to my son James, as soon as he shall be of the age of Twenty-one years, and Do to be given to my daughter, Susanna, as soon as she arrives at the age of eighteen, or gets married, to the satisfaction and with the approbation of Mary Rumsey, Edward Rumsey, Mary Morrow and Joseph Barnes; one other share to be given to my daughter Clarissa, at the same age, and under the same restrictions of Susannah, and the last share or fourth to come to Edward Rumsey, Junr, at my death.  I also ordain him my executor of this my will, The children to be Educated in as ample a manner as the income of their estates will allow.  In Testimony of this my will I have hereunto set my hand and affixed my seal this fifteenth day of May in the year of our Lord one thousand seven hundred and eighty eight. James Rumsey. Witnessed in the presents of Benjamin Wnykoop Joseph Wynkoop. Proved and recorded in the office of George Campbell, Esq., Register of the probate of wills in the City and County of Philadelpia

p. 538

26 March, 1793.  Isaac Wampole D. Regr.  Letters of adm. issued to Edward Rumsey, Jr. This will duly authenticated by the Register of the City of Philadelphia was presented to the Court of Berkeley Co. "By Edward Rumsey, the exor therein named, who made oath thereto, according to law, and ordered to be recorded; certificate is granted him for obtaining a probate thereof in due form, Given security with Nicholas Orcutt and Smith Slaughter, who entered into and acknowledged bond in the penalty of 2000 pounds, conditioned for his true and faithful admn. of said Decedent's Estate."

2.  W.S. Laidley.  1992.  "James Rumsey—The Inventor of the Steamboat."  Pages 222-227 in Genealogies of West Virginia Families: from The West Virginia Historical Magazine Quarterly, 1901-1905.  Genealogical Publ. Co., Baltimore, MD (online at GenealogyLibrary.com).  On p. 224:

p. 224

Charles Rumsey came from Wales in 1665, and settled at the head of Bohemia river, in Cecil Co., Maryland.  He married Catherine _____, Sept. 26, 1675.  She died in 1710.  Their children were: Prudence, born 1679; Margaret, 1682; Grace 1685; Elizabeth, 1687; Mary, 1692; Charles, Jr., 1695; William, 1698; Edward, 1703. Edward married Miss Douglas, from Scotland, and their children were:  Edward, Jr., Charles, Margaret, Susan, and Mary. Edward, Jr., married Anna Cowman, or (sic) Maryland, and their children were Charles, James, and Edward the 3rd. James Rumsey was born in 1742-3 and died in 1792.  The exact dates according to our calendar, are March, 1743, and died Dec. 21, 1792, in London.  He married a Miss Morrow, and their children were James, Jr., Susan, and Clarissa. James, Jr., through disease, became deaf and dumb. Susan married, first, Mr. Fraley and, second, Jacob Skiles. Clarissa married Geo. D. Minor, of Bowling Green, Ky...

p. 226

James Rumsey had a brother-in-law, Joseph Barns, who was a carpenter and who made the boats for Rumsey.  Joseph had a daughter, Polly Barns, whose mother was a sister of James Rumsey.  Joseph Barns, it is said, went to London to look after Rumseys affairs after his death and was never heard from afterwards...

Laidley redundantly tells us Joseph BARNES is James's brother-in-law — after having stated above that his only siblings were two brothers.  Joseph BARNES is mentioned in James's will, so he likely is James's brother-in-law

3.  Worth S. Ray.  1950.  Tennessee Cousins: a History of Tennessee People.  Self-published, Austin, TX (online at GenealogyLibrary.com).  On p. 288:

CAPT. CHARLES MORROW, SON OF REV. DAVID MORROW, OF CAROLINE CO. VA.  CAPT. CHARLES MORROW was Captain of a company in the revolution, that belonged to the regiment commanded by COL. PHILLIP SLAUGHTER.  We have been unable to identify the wife of CAPT. MORROW, but they were parents of one child and daughter, MARY MORROW, who became the wife of JAMES RUMSEY, the inventor of the first steamboat, who was the son of an EDWARD RUMSEY, who settled on BOHEMIA RIVER in Cecil County, Maryland, in what was called "BOHEMIA MANOR", the home for nearly a generation, of some of the MORROWS, who moved there from Virginia, for the benefit of the classical school which had early been established.  CAPT. CHARLES MORROW spent a good part of his fortune "backing" his son in law JAMES RUMSEY, who died in England, while there in connection with his invention.  He was buried in ST. MARGARET'S, Westminster, in LONDON. - 24 wm. & Mary pp 154, etc.

4.  Encyclopædia Britannica CD 98:

In the eastern United States James Rumsey, the operator of an inn at the Bath Springs spa in Virginia (later West Virginia), sought to interest George Washington in a model steamboat he had designed.  On the basis of Washington's support, Virginia and Maryland awarded Rumsey a monopoly of steam navigation in their territories.