Given facts:

1. P is the brother of Q and R. → P, Q, R are siblings; P is male.
2. S is R's mother. → S is a parent of R.
3. T is P's father. → T is a parent of P.

From (1) and (2): R, Q and P are siblings and S is mother of R. In standard family-relationship puzzles siblings share the same parents unless stated otherwise. Therefore S is mother of R, and hence also mother of Q and P.

From (3) and sibling relation: T is father of P, and since P, Q, R are siblings, T is the father of Q and R as well.

Now check each statement:

I. S and T are a couple.

• True. S is mother (parent) and T is father (parent) of the same children $\Rightarrow$ they are a parental couple.

II. Q is T's son.

• Not definitely true. We know Q is a child of T, but Q's gender is not given. Q could be son or daughter. So this is not definitely true.

III. T is Q's father.

• True. From T being P's father and P and Q being siblings, T is father of Q.

IV. S is P's mother.

• True. S is mother of R and R is sibling of P; hence S is mother of P.

V. R is T's daughter.

• Not definitely true. R is a child of T, but R's gender is not specified. R could be son or daughter.

VI. P is S's son.

• True. P is male (given "brother") and S is mother of P (from above), so P is S's son.

Count of definitely true statements = I, III, IV, VI = 4.

Therefore correct option: C. Only four.