I think I would take it as the father's wife (it is the father's wife and NOT the bloodline mother)
So (in my opinion)
-your significant other = 7th house
-your significant other's father = 4th from the 7th
-your significant other's father's wife = 7th from that 4th
= the 4th house of the chart (ie, 4th house from the 1st house on the chart wheel)