Don't worry, she's not a real mother, just "mother". Here's the story summary (copypasta'd of course):
1000 years ago, there lived a being known as "Enka". Enka tried to destroy and massacre the human race until finally everyone in the world worked as one to finally defeat him. As Enka died, he last muttered, "My child shall awake in 1000 years and finish what I started." Now in year 2063, many possible suspects, thought to be Enka's child have been found and gathered, but even with the newest technology it it still impossible to figure out who the child is. The Japan Supernatural Phenomenon Provision then came up with a plan. This plan was named the "Peaceful Family Project". The purpose, to find out who is the true child of Enka and teach them about the love of family and stop them from destroying the world. Ouka Midarezaki is assigned to be the father in the family. His wife Kyoka (a possible child of Enka) and the other 5 children start their unnatural life together.