I don't think Tatiana feel religion as a mere duty and went to church only to please her parents. She was less expressive than her elder sister, and much less talkative, so , people tends to thing she was cold and not as sensitive as Olga was. No proof about it. Maybe Tatiana was shy, like her mother, or had some difficulty to show her real emotions and toughts, but I think we can't said she didn't feel religion as deep as her sister. Olga and Tatiana personalities were very different, but both of them were religious...on their own way.
RealAnastasia.
Very well said RA. I could really relate to her with that. It was Mr. Gibbs observation on the girls' religiosity in his own perspective. Being religious/spiritual doesn't need to be overly expressed for the sake of others, but rather a personal/one-on-one relation with The One above in the depths of one's heart. Maybe that's what Tatiana was, she may seem looked cold outside but very warm and loving inside. And on her being the one who her mother chooses to pray with the most, is because she's the more favorable(or who's always with her mother) than the other girls, as we already know.