Are they married on that 1891 census?
How old were they in 1891?
Their ages and whether they travelled together or not and were they accompanied by family is important to know.
I assume Jane Turnbull's father is a brother to William and John Turnbull.
If you know the 'father's" name try searching the deaths using that father's name. My bet is it's probably William or John which isn't an advantage. Perhaps their mother had an unusual name!
It's a big country and attracted a tremendous amount of new chums into every state. You have yourself a bit of a needle in a haystack.
Make a list of what you know about both; age, occupation, marital status, religion etc. then throw it out there.
Any chance you can find death certificates of Jane's father's parents. They might be mentioned on that.

Good advice from janilye.

Its impossible to locate people without approximate birth year and parents names. Even then it's not always easy to find people because the information appearing on our records vary from state to state in Australia. Include what details you can and we will start searching the records.

Do you have any information to confirm they actually moved to Australia?

First, I want to thank janilye and tonkin for their excellent and thoughtful replies.

I know this is a needle-in-a-haystack situation.

William and John were the sons of John Turnbull and his wife the former Martha Goulding. John Turnbull - the father - was born 1826 in Cumberland, England, and died 1886 in Cumberland. He married Martha in 1851 in Cumberland and Martha was born 1829 in Cumberland and died in 1915 in Cumberland.

William Turnbull was born 1862 in Wigton, Cumberland, and married Margaret Sewell in 1890 in Carlisle, Cumberland. In the 1891 census they showed no children. William was a general labourer.

William's brother John Turnbull was born in 1865 in Carlisle and was a general labourer. The 1891 census lists him as single.

Neither William nor John appear in any census after 1891. It is my guess they moved to Australia in the 1890s.

My grandmother - niece to William and John - always said that two of her uncles emigrated to Australia and these two persons are the only possibilities. My grandmother did not know if there was ever communication from them once they reached Australia. She said both had been members of the Salvation Army. I do not know if the brothers travelled together but would guess that William and his wife Margaret would travel together.

I have no actual proof - other than my grandmother's story - that the brothers arrived in Australia. With best wishes, Traveller

We could try tracing William's wife Margaret.
This one is the daughter of Thomas and Jane Sewell;
Baptism
Name: Margaret Ellen Sewell
Gender: Female
Baptism Date: 17 Apr 1864
Baptism Place: Christ Church,Carlisle,Cumberland,England
Mother: Jane Sewell
FHL Film Number: 90587, 90588
Census 1881
NAME: Margaret Sewell
AGE: 24
ESTIMATED BIRTH YEAR: 1857
RELATIONSHIP TO HEAD: Daughter
FATHER: Thomas Sewell
MOTHER: Jane Sewell
GENDER: Female
WHERE BORN: Carlisle Cumberland England
CIVIL PARISH: St Mary Within
COUNTY/ISLAND: Cumberland
COUNTRY: England
STREET ADDRESS: Scotch Street
REGISTRATION DISTRICT: Carlisle
ED, INSTITUTION, OR VESSEL: 4
NEIGHBOURS: View others on page
PIECE: 5157
FOLIO: 56
PAGE NUMBER: 3
marriage registration
Name: Margaret Sewell
Registration Year: 1890
Registration Quarter: Oct-Nov-Dec
Registration district: Carlisle
Parishes for this Registration District
Inferred County: Cumberland
Volume: 10b
Page: 834

Yes, that must be her. But I don't think she used the middle name Ellen. I don't see her as an immigrant on the Ancestry site.

I fear that I may have made a fundamental mistake, at least with William Turnbull. On doing further checking I find that he died in England in 1916 and that it is highly unlikely that he ever went to Australia. I am very sorry to have put others to work on this blind alley. It has been a learning experience. Again, sorry.