You need to have a look at helen81_1's tree to see what was going on. Lots of certs on there. It is nice to see truthfullness on certs for a change!! Frank and Harry were different men.
Your clue is on young Bessie's death cert in 1918 - the death was certified by A.P. FARRELLY, Aunt, of Mackay. The relationship is to the deceased, so she is the child's Aunt.
I believe this was Alice Phoebe AVENELL who married Thomas Walter FARRELLY in 1908. The relationships are a bit complicated but you can work it out with the BDM online (I haven't really looked much but I don't think the births were all registered) and the newspapers. You need to read this article part way down the second highlighted column - Three weddings in one family....
http://trove.nla.gov.au/ndp/del/article/80278235George BRYAN's mother was Sophia AVENELL formerly BRYAN nee SACHAN (or something similar). Alice Phoebe was his half sister, mother Sophia and father Henry Arthur AVENELL, and Mary AVENELL was Alice's full sister.
Sophia SACHAN married firstly to Charles BRYAN in 1873, and secondly to Henry Arthur AVENELL in 1894.
I think your Harry is a full brother to George but working out the children is a bit of a mission. Just put BRYAN or AVENELL in the surname box for a birth search and scroll through as the forenames differ a bit on the entries. Sophia may have had a number of children between the death of Charles BRYAN in 1883 and her remarriage in 1894.
This looks like your Harry's death:
1926 C679
Henry BRYAN
Father: Charles BRYAN
Mother: Sophia SACHAN
Henry AVENELL and George Detley BRYAN were the executors of his Will.
http://trove.nla.gov.au/ndp/del/article/168808170Harry's birth?
1880 C3808
Unnamed (Male)
Father: Charles BRYAN
Mother: Sophie SACHAN
I would be getting Sophia's death cert to get a list of all her children.
Debra