Is a wife permitted to prevent her husband's father from touching her breasts, which he pulls out from the opening of her jilbab and squeezes forcefully, based on his understanding that a woman's 'awrah in front of her maharim is from the navel to the knee, and is she considered sinful if she prevents him from doing so?

The preponderant opinion is that what a man is permitted to look at of his mahram relatives is what typically appears, such as the face and extremities. He is not permitted to look at the chest and breasts. This is the view of the Maliki and Hanbali schools of thought. Furthermore, the permissibility of looking and touching is restricted by the absence of fitnah (temptation/strife) and the non-existence of lust. Accordingly, it is not permissible for the husband's father to touch the wife's breast, and she must prevent him from doing so.