Anthony,
I see 4 likely siblings to your John:
Henry bapt 1st May 1858
Mary bapt 27.2.1860 sponsors Charles & Mary Conway
Sarah bapt 15.6.1865. Parents address 16 Little George St. Father a sawyer.
Peter born 12.5.1869. Parents address 49 Mill St, Belfast. Father’s occupation sawyer.
The 1858 & 1860 events are on the rootsireland site. The 2 later births (which are after the start of statutory birth registration) are on the irishgenealogy siye:
https://civilrecords.irishgenealogy.ie/churchrecords/images/birth_returns/births_1865/03576/2317967.pdf
You could search that site for John & Sarah’s deaths, 1869 onwards.