Q: What is the Islamic ruling when a men marries a divorced or widowed women with a daughter who is not his own? What is the Islamic ruling when the same man has two boys of his own when he marries the above women?
A. The daughter of the woman he married is his stepdaughter. He is her Mahram, so when she becomes Baaligh, the law of Hijaab will not apply fully. This means that she may come in front of him and speak to him, but they must not travel together without her mother, nor should the girl be alone with the stepfather, unless such a situation arises through circumstances beyond their control. The stepfather should maintain her and treat her as his own daughter. She will not inherit from him when he dies or him from her when she dies.
The boys will make Purdah from this girl when they are Baaligh, because they are not Mahram to each other. Likewise the boys will not inherit from their step-mother.
And Allah knows best.
Mufti Siraj Desai