Does "My wife is as unlawful to me as my sisters" constitute Dhihar if the husband says it to himself secretly to deter himself from a certain act, with his wife being unaware of it? And is a wife kissing her husband's private parts forbidden?

If a man says to his wife: "You are unlawful to me like the unlawfulness of my sister or my mother," then, according to the preponderant opinion, he is a mutazhahir (one who commits zhihar). No divorce occurs by this statement unless he intends divorce. Rather, he is bound by the kaffarat az-zhihar (expiation for zhihar), which is freeing a slave. If he cannot find one, then fasting for two consecutive months. If he is unable to do that, then feeding sixty poor people. It is not permissible for him to have intercourse with his wife before performing the expiation. Zhihar can be made conditional, and it occurs upon the fulfillment of the condition. The wife's lack of knowledge of what the husband uttered is not taken into consideration. Zhihar is indeed an abominable saying and a falsehood.