Complex sentence: One day Yousuke found the couple that adopted his sister, the problem is that they have two daughters, Ayu and Mayu, both of which were adopted from the same orphanage and are roughly the same age.
Split into simpler sentences: One day Yousuke found the couple that adopted his sister. The problem is that they have two daughters, Ayu and Mayu, both of which were adopted from the same orphanage and are roughly the same age.