Answer:

There is no harm if your Mother uncovers her face in front of her husband’s children whom he had from another wife, and this is because they are from her Mahrams, Allaah The Most High said: (24,31) (أَوْ أَبْنَاءِ بُعُولَتِهِنّ) […Or their husband’s sons (24,31)]

[source: Al-Lajnah ad-Daa-imah [Fataawa almar-ah al muslimah–daar at-tawheed p.751]

Translated by: Abu Zainab (Muhammad Al-Hindee)