How is the issue of the stepdaughter's hijab from a previous husband dealt with when the husband has children from a previous wife living in the same house after they have reached puberty?

Your wife's daughter from another man is a stranger to your children from a different mother, and she must be treated as such. Therefore, there should be no private seclusion (khalwa) and no unlawful mixing. She must observe hijab in front of them. For them to live in the same house, it is stipulated that the house must be spacious, and they must not share facilities. If these conditions are not met, and necessity dictates that they live together, then caution must be exercised, private seclusion must be avoided, and the girl must adhere to modesty. Necessity is defined as a person reaching a point where, if they do not commit the forbidden act, they would perish, or be close to perishing, or fall into a hardship that is difficult to bear.