With regard to getting certificates -- If you go back a generation in my family, a marriage certificate gives the bridegroom's father's name --- except that there was no such person!!!!!!!
The information on any bdm certificate is only as accurate as the knowledge of the informant.  Of course it is your choice whether or not to purchase an online certificate to validate the research.  Surely there was other identify information on your marriage certificate ie if it was an Australian Certificate.  
Information on 
Victorian Marriage Certificates
 Date and place of marriage;
 name and surnames;
 conjugal condition, whether bachelor or spinster, widow or widower,
 stating date of decease of former wife or husband;
 children by each former marriage;
 birthplace; occupation;
 age; residence (present and usual);
 parent's names of each party (including mother's maiden name) and occupation of father; signatures of the parties and witnesses;
 signature of Minister. Prior to 1963 the date of termination of previous marriages of  the parties is also shown.
The children born to Joseph and Jane are - [I missed a few due to spelling variations of the surname...thanks Essie 

]  And note that Mary J was the third dau....so it would appear that Marcella was an elder sister.  
If you purchase any of the following birth certificates it will show the names of the previous children of the marriage and their ages.  Also note the usual 2 year gap between most of the births.  I think Marcella was born either 1871 or 1872 and her age at death is incorrect.  Probably correct on that marriage cert as she would have needed parental consent to marry in 1889 even if born c1869.
Births
GARRETT Thomas
Father Joseph  Mother Jane MURTHA
At Cowwarr  1873  Reg#15862          Died Cheltenham  1953  79 years   Reg#4072
GARRETT Ellen      
Dau# 2Father Joseph  Mother Jane MURTHA
At Cowwarr  1875  Reg#1659 
GARRETT William
Father Joseph Mother Jane MURTHA
At Cowwarr 1877  Reg#8152              Died Hotham  1931    54 years  Reg#9839   
GARRET Joseph
Father Joseph  Mother Jane MURTHA
At Cowwarr  1879  Reg#15279 
GARRET Mary   
Dau#3 according to the marriage notice in 1900.
Father Joseph  Mother Jane MARTHA
At Cowwarr 1881  Reg#8557      Died as Mary McGRATH at Bentleigh  1962 81 years Reg#9688
GARRETT Eliza
Father Joseph  Mother Jane MARTHA
At Cowwarr  1883  Reg#8423   Died as Eliza McGRATH at Fitzroy 1963  80 years Reg#22750
No birth registration
GARRETT Gregory born c1885.  Died 1960 at Werribee aged 75 years.  Reg#26435
Parents Joseph GARRETT and Jane MURTAGH.
GARRETT Edward Stephen
Father Joseph  Mother Jane MURTHA
At Cowwarr 1887  Reg#27607              Died at Sale  1946 58 years  Reg#17366 
GARRETT James
Father Joseph  Mother Jane MURTHA
At Cowwarr  1890 Reg#31620              Died 1970 Bunyip  79 years Reg#20827
I suggested you view Joseph's and Jane's probate file available online but I have just been advised by the Public Records of Victoria that the digitised wills will probably not be available again until next week.  It is a public holiday in Victoria tomorrow however their techs are working on the problem now.
http://nla.gov.au/nla.news-article59436344Administrators of Joseph's estate selling machinery and stock.
http://nla.gov.au/nla.news-article67395565Application for probate by the executors of Jane's Will.
http://nla.gov.au/nla.news-article71544491   - ?too much Christmas cheer 
Traralgon Record and Morwell, Mirboo,Toongabbie, Heyfield,Tyers and Callignee Advertiser 25 Jan 1884
TOONGABBIE POLICE COURT.
At Toongabbie Police Court on Tuesday, before a bench of magistrates, Joseph Garrett was fined 10s for insulting behaviour at Cower on last Christmas Day.
Either Joseph's or Jane's death certificate will list all the chn of the marriage both living and deceased at the times of their respective deaths.
Cheers
Cando