Praise be to Allah.
It is not permissible for a woman to allow her husband’s father to have his way with her under any circumstances whatsoever, even if she has to fight him off. She must keep away from him and put whatever obstacles between her and him will protect her from him and his evil, when she realizes that he has any unlawful inclination towards her, or any kind of infatuation with her – Allah forbid.
It is not permissible for her to be alone with him; even if she is not alone with him, she must wear concealing clothing. In many cases of sexual harassment between mahrams, the cause of it is carelessness with regard to uncovering ‘awraat in front of them. So you will find a woman wearing tight clothes which show her legs and arms and more than that, on the grounds that she is sitting with her mahrams. She does not realize that the Shaytaan is always trying to tempt people to do everything that sis haraam.
If a man commits zina with his son’s wife, and forces her to do that, then he deserves to be executed, because he has committed zina with a woman who is a mahram to him. Allah, may He be exalted, says (interpretation of the meaning):
“Prohibited to you [for marriage] are your mothers, your daughters, ... And [also prohibited are] the wives of your sons who are from your [own] loins, and that you take [in marriage] two sisters simultaneously, except for what has already occurred. Indeed, Allah is ever Forgiving and Merciful”
If a woman is a mahram, then committing zina with her is even worse in terms of sin, and is a worse crime than committing zina with someone else to whom marriage is permissible. Hence the punishment for zina with a mahram is execution in all cases, whether the culprit was previously married or not, according to the correct scholarly view.
You can refer the case to the court in your country, even if they do not judge in accordance with Islamic law – as you are in a non-Muslim country – and they may punish him in a way that deters him from attempting to repeat this offence.
With regard to your marriage to the son of this rapist, it is not rendered invalid according to the more correct scholarly view.
See also the answer to question no. 127992 .
And Allah knows best.