I think that the parents of Louis Fourie are actually Pieter Fourie b11 and Isabella Steyn and not Petrus b16 and Isabella.
I also do not think that Isabella was ever married to Petrus b16.
DP vol 1 page 229 and 231
The mystery deepens.
I checked SAG . There Petrus is changed to Pieter at d16 , Married to Isabella and children as indicated previously under pieter at d11.
Genealogy of the Fourie Family site also agrees with SAG.
It therefore seems that i and DP are wrong.