The Qur’an encourages us to establish good relations with all people. This injunction would include those who do not follow the Islamic creed. More importantly is to be good to relatives or those who are considered Mahrams such as stepdad.
Regarding the issue of the stepdad (mother’s husband), he becomes a mahram to his new wife’s kids so they are are like his children. Which means the daughters for instance, have the freedom not to wear hijab before him. Also they are recommended to treat him in a good way and he should treat them as his children. Children in this situation should however bear in mind that he is not their father, so they are not required to obey him the same as they obey their father. They have the option to accept his suggestions or politely and kindly refuse them. This does not mean to deal with him unfairly, but they won’t be punished if they do not carry out all his orders.