Going on your dates of late 1700's to early 1800 you need to look in NSW (New South Wales, Australia)
If you google NSW BDM and search for your name variations you will find some records.
V18174359 1B/1817 SIBREE JOSEPH (f)JOSEPH (m)SARAH
V18184662 1B/1818 SIBREE REUBEN (f)THOMAS (m)SARAH
Also google NSW State Records and use 'keyname search' PLUS go to Indexes Online and check our Colonial Secretary Records.
TROVE (nla newspapers) could have notices for them.