Read the following passage and solve the questions based on it.
There are four members in a family—Kitto, Litto, Mitto and Nitto. Among these four persons, there is one couple, their son and their daughter. When asked about their relationships, the following were their replies:
Kitto: Nitto is my husband. Mitto is my daughter.
Litto: Kitto is my mother. Mitto is my son.
Mitto: Kitto and Litto are of the same gender. Litto is my sister.
Nitto: Litto is of the same gender as I. Mitto is my son.
It was known that only one of them always speaks the truth.
Among the four, who cannot be the truth teller?