Is muslim, who has been raised by a stepmother (his mother died shortly after divorce of his parents, when he was 4 years old), and who shows her respect as for his biological mother, not thinking about his late biological mother - commiting sin? The stepmother is loving him as her own child, but there is no blood ties between them.

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and Messenger.

A person is not sinful for respecting and loving his step-mother who is kind to him, rather he will be rewarded for doing so. It is reported in an authentic Prophetic narration that the one who does not thank the people who did good to him, does not thank Allaah. Allaah Says (what means): {Is the reward for good [anything] but good?}[Quran 55:60]

It is also known that a person is naturally inclined to love those who are kind to him. Additionally, she deserves kindness since she is the wife of your father. However, a Muslim should not forget his biological mother who died, rather he should supplicate for her as Allaah ordered us to supplicate for our parents. Allaah Says (what means): {…and say, “My Lord, have mercy upon them as they brought me up [when I was] small.”}[Quran 17:24]

For more benefit, please refer to Fatwa 90201.