I'm probably going to raise more questions here than answers
Firstly, as you have found, there doesn't seem to be a marriage for Ralph and Mary Watson. Their first child Eliza b. 1865 shows as illegitimate on her registration with both parents present....so, they may never have married. You would need to look at the other birth cert for Janet to see whether any marriage details show on that one.
I don't know how much you have on the censuses for Ralph, so I'll include some here:
1851:Ralph Glasgow 42, Hand Loom Weaver Silk
Agnes Glasgow 45 (Agnes is a name variant of Ann)
Ralph Glasgow 15, errand boy
Alexr Glasgow 13
Margt Glasgow 11
Janet Glasgow 7
Address: 20 Old Dalmarnock Rd, Bridegton, Glasgow
1861:Ralph Glasgow 52, Cotton Weaver
Ann Glasgow 55
Janet Glasgow 17, daugh., Power Loom Weaver Cotton
Ralph Glasgow 25, son, flesher
Rosina Glasgow 5
Ralph Glasgow 3
John Glasgow 2
Address: 20 Old Dalmarnock Rd, Bridegton, Glasgow. All showing Glasgow birth places.
This looks a possiblity for daughter Isabella Jackson Glasgow b. 1860:
Isabella Glasgow 8 Mo, niece, b. Glasgow
Isabella Handrew 18, wife, b. Paisley, Renfrewshire
William Handrew 20, Cotton Yarn Dresser, b. Glasgow
Address: 2 Muslin St, Bridegton, Glasgow
From IGI, I think the surname might actually be Hannay. There is a marriage showing in 1860 between a William Hannay and Isabella Campbell. Can't easily see the 'niece' connection there (maybe adoption?).
1871:Ralph Glasgow 36, butcher
Mary Glasgow 35, b. Monross, Forfarshire
Ralph Glasgow 13
Janet Glasgow 2
Address: 31 East Miller St, High Church, Glasgow
Daughter Isabella b. 1860 does not seem to have fared well. By 1871 she is showing 'under detention' at the Glasgow Industrial Girls School:
Isabella Glasgow, 11, b. Glasgow
Address: 61 Rottenrow, Glasgow
As you, can't easily see a death for Elizabeth Jackson. Something looks to have happened to the family unit I would say following the birth of Isabella in 1860 and before the 1861 Census. I wouldn't necessarily assume that because Isabella doesn't show as deceased in son John's DC in late 1861, she hadn't died before this date.
Regards.
Monica