In the beginning of the anime we're lead to believe that Daikichi's grandfather and this Masako woman are Rin's parents.
During the anime they confirm that Masako is Rin's mother, but they leave the father's identity up in the air. If Daikichi's grandfather was Rin's father, he would have had to taken part in her conception at 73 years old - not impossible, but it's still a stretch.
Is Daikichi's grandfather really Rin's father? Is this revealed in the manga?