Father in law and daughter in law....

If a person divorces his wife once, he can take her back. 

if he divorces her twice, he can still get back to her.

if a person divorces his wife thrice, he can still get back to her after the halaala and other proceudres. 

But in this masla of Hurmate Musaaharat a person can never ever go back to his wife, she becomes haraam on him forever. 

and this happens in situations like these where the father in law touched the daughter in law with lust and desire (or vice versa). 

the same goes for the son in law and the mother in law. there are more sceanrios to this also, we should learn our deen and enquire from the learned Ulama.

A practise which is common amongst us, especially when families get together, is the shaking of hands. For example the father in law with the daughter in law. Although it is basically permissible, but due to the rage of todays prevalent fitnahs, they should abstain from even shaking hands. hugging and kissing is out of the question. in fact, he should not even look at her with interest, they should just make a verbal salaam.
If we look into the lives of our pious, we will find they would not even look at their daughter in laws, even though they stayed in the same house, under the same roof.

And with regards to this, our sheikh Hazrat Shah Hakeem Muhammad Akhtar Saab (Rahimahullah) gives a very beautiful example: The electricity department announce they will be doing some maintenance work in the area from a certain time till this time. but there is a condition added in there as well. "treat the wires as live at all times, because we might switch it on in between for testing purposes. you never know when the current will come on, if you happened to be in contact with the wires at that time, the outcome can be fatal.

You may be having a clean heart by shaking hands etc, but you never know what's in the mind of the other person. Or when Shaytaan may attack and spur the haraam desire you never now. Resulting in dangerous consequences.

Hence we should refrain from any physical contact.

May Allah Ta'ala give us all the Tawfeeq.

Hazrat Moulana Abdul Hamid Ishaq Saab Daamat Barakaatuh