Track in-law relations by linking through the spouse: spouse's father $\Rightarrow$ father-in-law; spouse's mother $\Rightarrow$ mother-in-law.
Step 1: Parent-child and marriages.
R has only two children, P and S (both daughters).
T is married to P.
Therefore, R is P's father.
Step 2: Relation to T.
Since T is P's husband, P's father (R) is T's father-in-law.
Also given "R is the son of Y" confirms R is male. \[ \boxed{\text{R is T's father-in-law}} \]