If you have sisters who are far apart in age, you can easily have the child of the older sister the approximate age of the younger sister. My father was only a couple of years older than his eldest sister's son, and they used to hang out together. It can happen if you have a big family, and the eldest seriously helps raise the youngest. It probably happened more often in the past when that type of a family was more common.
In this case, the blonde is still in college, while the redhead is obviously beyond that stage of her life. They look similar in age because that's the way the developers want it. At least they didn't use Honey Select or Koikatsu models, where the only way you can tell the MC's grandmother from his daughter is by hair color and glasses.