I think that dutta knows the truth that his sisters are evil.....as Kala's husband said that dutta is a smart guy. He knows everything about his enemies etc......there is no way that he doesn't know the real face of his sisters. I think he is quite because he loves them more that his life. Also, he might be quite because sister's mother has brought him like her own son. He doesn't want her to get hurt by knowing the truth.