Ririko dies young and leaves behind her 6 year old daughter, but she was just as irresponsible in death as she was in life and tells Meguru that her father is likely either Touma or Sentarou (probably) so just pick one of them to be her dad. And never get a DNA test! And so the three of them start living together, and bring up a lot of old memories, mostly for Sentarou.
I love everything about this. Meguru is such a sassy deadpan girl
Daraku Kazoku-ron