My father is not Muslim, my parents got separated and then my mother became Muslim. Since then they didn’t have contact. May I consider him my mahram? May I take off the hijab in front of him and kiss him?
You may consider your non-Muslim father as your Mahram, thus uncover (parts of the body that can be exposed in front Mahrams) in front of him.
Imam al-Kasani (Allah have mercy on him) states:
“A Mahram is one with whom marriage is permanently unlawful… whether this Mahram is a free person or a slave, for slavery is not contrary to close relationship (mahramiyya), and whether he is a Muslim, a non-Muslim or an atheist (mushrik), for a non-Muslim Mahram normally safeguards her, except that he is a fire worshipper, for he considers marriage with her to be permissible.” (Bada’i al-Sana’i, 2/124)
However, if there is a fear of lewdness, immorality or temptation (fitna) or you fear that by kissing him, he will have evil thoughts and desires, you must avoid uncovering in front of him and kissing him. This is regardless of whether one’s Mahram is a Muslim or non-Muslim, and the reason is obvious. The jurists (fuqaha) have stated that, covering becomes necessary even in front of Mahrams if there is a fear of Fitna or desire.
As such, it is good to be careful in this regard, but if you do not fear any fitna, then it would be permitted to remove your Hijab in front of him and kiss him.
And Allah knows best
[Mufti] Muhammad ibn Adam
Leicester , UK