Jackie,
From what I've found with the info. on 1891......
South Boisdale
Marion MacInnes Head W 62
Janet MacKay Dau M 24
Marion MacKay GrnDau 4
Alexander MacInnes Son 32
Seems to be.......
Marion MacInnes c 1832-1895 m/s MacDonald daughter of Alexander had Janet MacDonald/MacInnes c1856 South Uist (illegitimate)? who married Roderick MacKay.
Marion's Mother's name/maiden name would be on her death cert. hopefully?
EDIT
Marion MacDonald married Andrew (son of Alexander) MacInnes - (sorry no date or place)
Children of Marion & Andrew:
1. Alexander c 1863
2. Mary c 1864
Roderick MacKay b c 1862 Tabost, Lewis, married Janet MacDonald/MacInnes
EDIT Roderick was son of Duncan who was son of Norman
Child of Roderick & Janet:
Marion married, supposedly (1st) to Robert Crawford of Kintyre, Argyll (son of Archibald), had a son Edward c1911 (died young), was widowed & married (2nd) to Alexander MacDonald of Glebe House, Daliburgh, S/Uist.
Annie
ADDED........
The birth of son Edward would confirm the marriage date for 1st marriage if it did take place as Edward may have been illegitimate if she has proclaimed to be single on "2nd" marriage?
The marriage to Robert Crawford (if there was one) would confirm if this is the same person with same parents.