Anna (Greeff) WIlliams died in about 1829, based on the birth date of her last child and the known date (1831) of the second marriage of her widower. On the list of her husband's spouses, she appeared second, possibly because of alphabetic reasons. From that I deduce that WikiTree does not take into account the date of death of spouses when sorting marriages.
There are of course workarounds for this, and in this case I had the date of death as the date of the end of the marriage. On her profile it looks unnecessary, so some kind colleague removed it. I have now put in "before 1813" as the date of the first marriage, and hope that this will be tolerated.
But it would be nicer if WikiTree software could check spouses' death dates when sorting marriages.