I would like to know whether a woman has to take Hijaab in front of her uncle (father's step-brother).

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad is his slave and messenger as well as that of his family and all his companions.

All the brothers of your father whether they are full or half-brothers are considered your uncles and a woman does not need to observe Hijab in front of such relatives unless she fears temptation. Read our Fatwa: 84126.

Allaah know best.