I haven't found details of this marriage as yet, but we can see quite clearly in the 1900 census return that there was a young girl named Rosomond Belknap living with her grandparents, Cornelius and Rosamond Knibbs at Athens, Bradford County, Pennsylvania. Cornelius had just one daughter, so we can assume that she married a man with the surname of Belknap.
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From the census return for the inhabitants of New York taken on Feb 1892, we see the following:
Wm E Belknapp M 29 US Labourer
Elizebith Belknapp F 27 US
Rosemond Belknapp F 8 US
Wm E Belknapp M 5 US
Gertrude Belknapp F 3 US
Cornelias Belknapp M 1 US
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In 1900 we also see the following entry for William Belknap and his family, living at Elmira City, Elmira Town, Chemung, New York:
William E Belknap, Head, Married 17 years, June 1863, New York, Stationary Engineer
Elizabeth A Belknap, Wife, Married 17 years, Sept 1865, New York
Rosomond Belknap, Daugh, 0ct 1884, New York
William E Jr. Belknap, Son, Sept 1887, Pennsylvania
Gertrude F Belknap, Daugh, June 1889, Pennsylvania
Cornelius Belknap, Son, Nov 1891, New York
Joseph H Belknap, Son, April 1894, New York
Harrison L Belknap, Son, Oct 1896, New York

Elizabeth had 7 children, 6 of whom survived.

However, we also see from the 1900 census at Athens, Bradford County, Pennsylvania.
Knibbs Cornelius Head 66 Labourer (Bridge Co.) England
Knibbs Rosamond C Wife 62 Pennsylvania
Belknap Rosamond G.Daughter 15 Pennsylvania

So from this, it appears that William's daughter Rosamond was staying with grandparents.

Details of Elizabeth's age were found at

Tri-Counties Genealogy & History Sites

According to the USSSDI Joseph's last place of residence was New York, USA

According to the USSSDI, Harrison's last place of residence was Rocky River, Cuyahoga, Oio, USA