Assalamualaikum ustaz. Is a woman considered mahram to her stepfather?
Waalaikumussalam wrt. wbt.,
Alhamdulillah, praise and gratitude to Allah SWT for His countless blessings for us all. Praise and salutations to our beloved Prophet Muhammad PBUH, his family, companions, and all those who follow his footsteps until the Final day.
The terminology mahram according to jurists means: “A person who is prohibited for us to marry with and the prohibition may be in the form of al-ta’bid (forever) for mubah reasons and not because of honour or there is something that prevents it that may be removed.” 
The explanation regarding the group of people who are considered mahram is stated in the statement of Allah SWT:
حُرِّمَتْ عَلَيْكُمْ أُمَّهَاتُكُمْ وَبَنَاتُكُمْ وَأَخَوَاتُكُمْ وَعَمَّاتُكُمْ وَخَالَاتُكُمْ وَبَنَاتُ الْأَخِ وَبَنَاتُ الْأُخْتِ وَأُمَّهَاتُكُمُ اللَّاتِي أَرْضَعْنَكُمْ وَأَخَوَاتُكُم مِّنَ الرَّضَاعَةِ وَأُمَّهَاتُ نِسَائِكُمْ وَرَبَائِبُكُمُ اللَّاتِي فِي حُجُورِكُم مِّن نِّسَائِكُمُ اللَّاتِي دَخَلْتُم بِهِنَّ فَإِن لَّمْ تَكُونُوا دَخَلْتُم بِهِنَّ فَلَا جُنَاحَ عَلَيْكُمْ وَحَلَائِلُ أَبْنَائِكُمُ الَّذِينَ مِنْ أَصْلَابِكُمْ وَأَن تَجْمَعُوا بَيْنَ الْأُخْتَيْنِ إِلَّا مَا قَدْ سَلَفَ ۗ إِنَّ اللَّهَ كَانَ غَفُورًا رَّحِيمًا
“Prohibited to you [for marriage] are your mothers, your daughters, your sisters, your father’s sisters, your mother’s sisters, your brother’s daughters, your sister’s daughters, your [milk] mothers who nursed you, your sisters through nursing, your wives’ mothers, and your step-daughters under your guardianship [born] of your wives unto whom you have gone in. But if you have not gone in unto them, there is no sin upon you. And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful.” 
The prohibition of marrying a certain woman is divided into two. The first is muabbad that is a person who is prohibited to be married forever. The second is tahrim muaqqat which is someone who is prohibited to be married to for a temporary time due to certain factors that prevent it. If it is still done, then it is included as a batil marriage and it is invalid.
According to the above question regarding mahram, the aurah and khalwat issue, a stepfather is a mahram mu’abbad (forever) to the stepdaughter, after the intercourse of the stepfather and mother after they are married. This is based on the statement of Allah SWT:
وَرَبَائِبُكُمُ اللَّاتِي فِي حُجُورِكُمْ مِنْ نِسَائِكُمُ اللَّاتِي دَخَلْتُمْ بِهِنَّ
“…and your step-daughters under your guardianship [born] of your wives unto whom you have gone in.” 
Thus, the aurah between rabibah (stepdaughter) in front of the stepfather is the same as the aurah between a daughter and her biological father, that is between the navel and knees if it doesn’t lead to fitnah and desires. Some scholars of madhhab Syafie limit the body parts that are usually visible when working, that is their aurah is all of their body except their head, neck, hands up to their arms and their legs until their knees. Likewise, the issue of khalwat doesn’t arise if they stay in the same house, just like other mahrams.
The author of the book al-Fiqh al-Manhaji  it is stated: “The aurah in front of a mahram man is between the navel and knees. This means it is permissible for them to expose other of their body parts on the condition that it doesn’t lead to fitnah. If there is a possibility of fitnah, then it is prohibited. This follows the statement of Allah SWT:
وَلَا يُبْدِينَ زِينَتَهُنَّ إِلَّا لِبُعُولَتِهِنَّ أَوْ آبَائِهِنَّ أَوْ آبَاءِ بُعُولَتِهِنَّ أَوْ أَبْنَائِهِنَّ أَوْ أَبْنَاءِ بُعُولَتِهِنَّ أَوْ إِخْوَانِهِنَّ أَوْ بَنِي إِخْوَانِهِنَّ أَوْ بَنِي أَخَوَاتِهِنَّ أَوْ نِسَائِهِنَّ
“And not expose their adornment except to their husbands, their fathers, their husbands’ fathers, their sons, their husbands’ sons, their brothers, their brothers’ sons, their sisters’ sons, their women,” 
The word adornment in the verse means the place the adornment is the body part in the upper part of the navel and below the knees.
Lastly, may Allah SWT ease all of our daily affairs in understanding and learning Islam. We end with the supplication:
رَبَّنَا هَبْ لَنَا مِنْ أَزْوَاجِنَا وَذُرِّيَّاتِنَا قُرَّةَ أَعْيُنٍ وَاجْعَلْنَا لِلْمُتَّقِينَ إِمَامًا
“Our Lord, grant us from among our wives and offspring comfort to our eyes and make us an example for the righteous.”
 See Kasyifah al-Saja, pg. 114
 Surah Al-Nisa’: 23
 See al-Fiqh al-Manhaji, 1/125
 Surah al-Nur: 31