Q: My sister remarried, her daughter is now 3. Can her husband (daughter’s step father) be a mahram to her once she is baaligh?
A: The daughters of the woman he married are his stepdaughters. He is their Mahram, so when they becomes Baaligh, the law of Hijaab will not apply fully. This means that they may come in front of him and speak to him, but they must not travel together without their mother, nor should the girl be alone with the stepfather, unless such a situation arises through circumstances beyond their control.
Allah Ta’aala knows best.
Mufti Siraj Desai