I am interested in finding where William's siblings went in Australia - there are a couple by the name of Charles Powell Guest and I'm not sure whether either is a brother.
Without the marriage certificate or death certificate of Charles Powell GUEST, you cannot be certain he is the brother of your great Grandfather William George GUEST.
This would seem to be his death.
11479/1932 GUEST Charles P
Parents William & Maria (incorrect father. Informant confused perhaps)
At Canterbury
However regarding the confusion of more than one Charles Powell Guest.
Charles Powell GUEST was married to Rebecca (probably nee MEADLEY)
She had apparently been married previously to Daniel or David ELDRIDGE,as a young bride, and married to GUEST under the name ELDRIDGE.
2417/1873 GUEST Charles
ELDRIDGE Rebecca
At GOULBURN
Among their children was Charles Ernest
14365/1880 GUEST Charles Ernest
Parents Charles & Rebecca
At Goulburn
Born to this son and Margaret J. was-
4582/1909 GUEST Charles P
Parents Charles E & Margaret J
At Junee
And his death
18998/1965 GUEST Charles Powell
Parents Charles Ernest & Margaret Johanna
At Parramatta
This son, Charles Powell Guest married
4641/1937 GUEST Charles Powell
MADDEN Lillian Mavis
At Burwood
Mavis died young only a year or so after the marriage. Here is her death notice-
http://trove.nla.gov.au/ndp/del/article/132278725 Here is an obituary for Rebecca, the wife of Charles Powell GUEST 1913
http://trove.nla.gov.au/ndp/del/article/142658081 Rebecca GUEST wife of Charles GUEST who died 1913 at Junee. It states her husband Charles is a business man of Junee. She was 69? (Hard to read ) From Asthma.
Her funeral below, states "will leave from her son-in law’s home", J. INGS. Her daughter Violetta married a man who, according to my findings was her cousin, James INGS
http://trove.nla.gov.au/ndp/del/article/142658028 Charles Powelll GUEST and his wife Rebecca spent some in in Melbourne between 1884 and 1889 where there was a birth and 2 infant deaths to them.
1918 deceased estate advice re. Charles Powell GUEST, bootmaker of Junee.
http://trove.nla.gov.au/ndp/del/article/15781557This man is possibly the immigrant Charles GUEST, husband of Maria nee TURNER.
You have not mentioned whether or not his middle name was also Powell.
Sue