All perfect praise be to Allah, The Lord of The Worlds, and may His blessings and peace be upon our Prophet Mohammad and upon all his family and companions.
Since a woman`s father-in-law is a Mahram, she is allowed to uncover the same parts she usually uncovers while at her own house, such as her hair, hands, feet and neck. Moreover, she is allowed to wear trousers before him and her other Mahrams.
However, this is conditioned with safety from temptation as well as the condition of the Mahram himself. If he has reached puberty and is likely to be tempted, or wasn`t religious and upright, then she isn`t allowed to show that which may cause more temptation as it is better to be safe than sorry.
Allah, The Almighty, Said in this regard (What means): "And say to the believing women that they should lower their gaze and guard their modesty; that they should not display their beauty and ornaments except what (must ordinarily) appear thereof; that they should draw their veils over their bosoms and not display their beauty except to their husbands, their fathers, their husband’s fathers, their sons, their husbands’ sons, their brothers or their brothers’ sons, or their sisters’ sons, or their women, or the slaves whom their right hands possess, or male servants free of physical needs, or small children who have no sense of the shame of sex; and that they should not strike their feet in order to draw attention to their hidden ornaments. And O ye Believers! turn ye all together towards God, that ye may attain Bliss" {An-Nur/31}. And Allah Knows Best.