Edward declared at the 1910 census return that he and Emily were married and had been for 20 years. I find that difficult to believe as we know he married his first wife, Elizabeth Knibbs, in 1871 and divorced her in 1888. At her divorce, Elizabeth stated that Edward's relationship with Emily started in about 1880, so I guess that's where the 20 years came from.

In 1871, he was lodging at 24 Vincent Square on the corner of Alfred Street, Westminster, London, having recently married Elizabeth:
Job Toop Head M 50 Cabproprietor Dorset
Mary Ann Toop Wife M 46 Somerset Bridgewater
Mary Ann Toop Daur 12 Middlesex Knightsbridge
John Doyleaf Lodger M 50 Carpenter Middlesex London
Emma Doyleaf Wife M 46 Middlesex London
George Doyleaf Son 12 Middlesex Westminster
Edward Hoare Lodger M 32 Corporal Royal Engineers Wales Merthyr Tydfil
Elizabeth Hoare Wife M 36 Surrey Egham
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Edward married Elizbeth Knibbs (aka opera singer Lizzie Lemure) in 1871 and they went off together to live in Chicago, Illinois, USA. They separated after about 10 years and eventually divorced in 1888 due to Edward's infidelity with Emily Knibbs, identified in the divorce as his niece.
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Whilst I haven't seen a record of any marriage between Edward and Emily Knibbs, we can see Edward and Emily in 1900 with three children Edna, Darrette and Lilian, living at Hudson, New Jersey, USA. The census states that they'd been married for 20 years.
Edward Hoare Head born Sep 1840 59 Married (20 years) Wales Civil Engineer
Emily Hoare Wife born Apr 1863 37 Married (20 years) England
Edna Hoare Daughter born Aug 18 1881 Illinois
Darrette Hoare Daughter born Jun 1890 9 New Jersey at School
Lillian Hoare Daughter born Feb 1893 7 England at School

Edward stated that he had been in the United States since 1870, and that Emily had been there since 1879.
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Edward and the children were living at Elizabeth Ward 10, Union, New Jersey in 1910, but Edward identified himself as a widower.
In 1910 we see her living at Elizabeth Ward 10, Union, New Jersey
Edward Hoare 69, ENG/Wales - civil engineer, widowed
Edna Hoare 28, ILL - nurse
Dorette Hoare 19, NJ
Lillian Hoare 19 [17] NJ

We don't know for sure that Emily was the daughter of Sarah Knibbs, but it seems most likely. I suspect she was born out of wedlock to Sarah and then raised in the family household where Sarah was still living at that time. She was listed in the 1871 census as Mary Ann's daughter.
Mary Ann Knibbs Widow 54 Lodging House Keeper Bow devonshire
Jemima Knibbs Daughter single 24 Machinist Egham Surrey
Sarah Knibbs Daughter single 28 Laundress Egham Surrey
Emily Knibbs Daughter single 8 Mile End Middlesex
Jemima Knibbs Niece single 4 Mile End Middlesex
Anna Farmer Lodger Widow 52 Silk Weaver Shoreditch Middlesex

Jemima is probably the daughter of another of Mary Ann's daughters, Frances Knibbs, so a grand-daughter rather than a niece as identified on the census return.
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Emily's aunt Elizabeth Knibbs married Edward Hoare in 1871 and went to live in Chicago, Illinois, USA with him. She then divorced him in 1888 due to his infidelity with an Emily Knibbs who was described at the court hearing as his niece. The belief is that it was this Emily Knibbs who went to live with them in Chicago at around 1880.
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Emily can be seen as Emily Hoare in on 15 Feb, 1894, arriving in New York aboard the ship named Richmond Hill from London, England. She was travelling with three children:
Emily Hoare, 30, a citizen of England, Unmarried
Edna Hoare, 11, a citizen of USA
Dorel Hoare, 3½, a citizen of USA
Lilian Hoare, 10 months, a citizen of England

The Richmond Hill was a small passenger ship with accommodation for approximatey 50 first class passengers. The manifest for that particular journey from London to New York shows that there were only 13 passengers onboard. Emily was in a second class cabin.

(Click image to see a larger version)

It's interesting to note that Emily was recorded on the passenger list as Emily K Hoare, so I presume the "K" was for "Knibbs"