Hi there
Can't see a marriage in 1888 for them on SP

Why not look up the birth cert of Alexander's birth in 1890 as this will give you Helen's maiden name and details of the marriage (date and place).
The 1891 transcript I am looking at has mistranscribed Helen's name:
David McPherson 29 iron moulder b. Hunghead, Stirlingshire
William McPherson 24 wife b. Stenhousemuir, Stirlingshire
Alex McPherson 1 b. Falkirk
Thomas Miller 21 - relationship to head shows as Bi. Could it be brother in law, brother to Helen?
Peter Miller 15 as above both Thomas and Peter show as born in Stonehousemuir.
Address: 25 East Bridge St, Falkirk
Monica