Note – the author of the light novel got carried away with the idea of having the sisters being a year apart in age. Based on the character birth dates given on the OVA website, there would actually be 2 calendar years between when the first and last girls were born in which a daughter could not have been born, and therefore some of the ages given are not consistent with the actual elapsed time. As an example, the youngest girl’s birthday (Asahi) is given as being in April. According to the light novel, Yōtarō joins the girls on 25 December, therefore Asahi is ~9 months old. By the time the events of the OVA occur (in early summer), Asahi would be over a year old and the eldest daughter would actually be 20 years old.