In Islam, a Muslim is encouraged to have an excellent relation with everyone, especially his relatives and his in-laws. That is why it is permitted in Islam for a man to call his father-in-law “father” or his mother-in-law “mother” as long as he does so to show courtesy, kindness, mercy, and compassion.
Sheikh Ahmad Kutty, a senior lecturer and Islamic scholar at the Islamic Institute of Toronto, Ontario, Canada states: “There is nothing wrong if a daughter-in-law or son-in-law calls his or her in-laws “father” and “mother” provided the intention is clear that they do so out of respect and love. But they should clarify the issue to anyone who may mistake them for their real parents.
The prohibition in the rulings of Islam does not apply to the above situation. It simply refers to changing the real identity of children following adoption.”