In a Hadith it is said that a woman should fear her brother-in-law like death.

Answer

The meaning of 'death' in the context of the Hadith is that the woman should fear her brother-in-law just as she fears death. That means, that she must stay far from him. Just as one would not play with fire or any danger, so too should a woman avoid coming even near to her brother-in-law. There is great fitnah in such contact. The purdah for brothers-in-law, male cousins, uncles-in-law and such close relatives is more emphasized.