For a spoilerish explanation:
A old woman with a young son drinks a potion that makes her younger but drank too much that it turn her into a baby.
The son unaware what became of his mother and confused why there is a baby girl(his mother) at his home, raise the baby girl as his own child.
Years later, the son whose now middle age, gets seduced by his teenage adopted daughter, unaware that it's actually his mother turned young.
After the time skip, the adopted daughter, now married to her adopted father, has a baby with him and pregnant with another child, regains her memories and realized she is actually her husband's missing mother.