Hi,
Did a little searching and perhaps have a lead for you to work from.
It does look like Robert James? sister Margaret was still in Belfast during 1915. There is a marriage listed in the LDS family search giving a Margaret Stevenson Morrison marriage date for the period Jul-Sep 1915, Registration District Belfast, Vol 1, Pg 248, FHL Film number 101263, Digital Folder 004199365, Image # 00108.
This does not however list the name of the groom. Unfortunately LDS family search has changed their indexing and record information so that it no longer is cross referenced by groom name (to the public any way), nor does it show the groom. A check at ancestry.com also does not show the groom, but lists other names that are on the page, one of these might be the groom but also may not be. Names listed were:
Agnes Russell
Margaret Stevenson Morrison
William Henry Irwin
Annie Flynn
James Fullerton
Agnes Lewis
Lillie Livingstone
Albert Edward Hall
William Stirling
Normally I would not suggest a ?pay for? site to obtain basic family structure data, however a situation where there are not Grandparents or Great Aunts or Great Uncles known to run questions by, warrants a different method. Ulster Historical Foundation has a site which seems reasonable for what your needs are and they are showing Margaret Stevenson Morrisons? marriage record. This should contain the name of the groom at least and will enable you to further track down this Great Aunt. I?ll put the long link in as it is keyed to her name and marriage date for the search.
http://www.ancestryireland.com/family-records/marriage-records-co-antrim-co-down/?search1=margaret&search2=morrison&yr=&dropdown=Antrim&spouseFirstname=&spouseSurname=&fatherFirstname=&fatherSurname=
You should also be able to obtain Charlotte and Alexanders Marriage certificate data from the above site as well. Registration was 1876, Registration District Belfast , Vol 11,Pg 366, FHL # 101252.
Even though an online search may show no further data listed, it is still worth sending for as sometimes there may be small details left out of the actual online index, that show up after receiving the actual record. You might be able to capture a middle initial or middle name which would help determine correct family lines.
It does appear that conventional Irish naming patterns were employed in naming of the children of Alexander and Charlotte (in the first names at least), so the names themselves might just lead to the correct family lines.
Irish Naming Conventions (A variation was fist name became the childs middle name, or middle name was mothers maiden name in at least one child if not more)
The 1st son was usually named after the father's father : John Alexander
The 2nd son was usually named after the mother's father: Robert James
The 3rd son was usually named after the father : William Charles Was Alexanders middle name William?
The 1st daughter was usually named after the mother's mother : Margaret with Charlottes Maiden
Given that Alexander was a Relieving Officer AND Assistant Registrar Belfast for Registrars Office for year 1901 B. M. D.s, I would carefully research the history of the position. Perhaps even contact the GRO directly to see if someone there might be able to assist you in information about ?one of their own registrars?. http://www.groireland.ie/contact_details.htm They might be able to assist you in determining through what years Alexander was an Asst. Registrar. According to the history here: http://www.groireland.ie/history2.htm the head was The Right Honourable Sir Robert Edwin Matheson, Registrar-General 1900 - 1909., Although he could have also been under head Thomas Wrigley Grimshaw, Registrar-General 1 September 1879 - 1900.
One question? Did William emigrate from Canada to Illinois, U.S.?
Hope this all helps and let me know if you come across other data that helps, Jeanette