The arrows showing the relationship between two people is shifted one step when the relationship involves different mothers. It doesn't happen when the mother is the same. For example: the relationship between X and Y.
A is the father of X
B is the father of A
C is a son of B
D is a son of C
Y is the wife of D
ie, A and C have the same father B but different mothers. There is an extra arrow 3 with no annotation from C to B. Arrows to X from A and B are correct. Arrows to B are shifted one step so there is an arrow from nowhere to Y.
Note that the relationship statement at the bottom is correct. I hope that is clear.
When A and C have the same mother and father then A and C are shown as siblings of each other, ie B is not involved and the diagram is correct.