So here is my further piece of advice -
A. Can you file defamation case or can that be done by your parents as well?
Answer - No. Defamation is filed only when someone is defamed publicly by accusing or labeling him or her to be someone who he or she is not or by putting allegations over him or her of such incident in which he or she is not involved.
Whatever is going on in your family is running privately in between you only and has not been informed or floated out in general by spreading it out to your neighbours or friends, business colleagues etc.
At max, what you can do is to file a criminal complaint of mental harassment but I will not advice you to do that as that is a futile exercise with almost no outcome. Police and courts in general do not bother about these sorts of complaint especially when that involves family members.
B. Your father wants to disown your brother. Can he do that?
Answer - Yes absolutely. He has all rights in this world to do that.
C. Should he disown your brother?
Answer - I will advice you in negative as once your brother has been disowned then by no legal process he can be re-owned. This way, he will be out from this family forever and ever.
Better option is to file a suit of eviction on the ground of domestic violence which I advised you last time. This way, he will be out from the scene of property for such time being only till everything becomes normal.
By normal I mean, either he divorces his current wife and comes back to live along with you guys in peace and harmony or his wife mends her ways to such an extent which is acceptable by all of you (something which is not likely to happen in any possible situation because she is a criminal and a criminal never mends her way).
And this will also serve your intention of not letting your brother's wife having any say in the property as once the son has been evicted from the property that is as good as disowning him. So, he will be left with no claim on any property whatsoever.
The only difference in between disowning him and evicting him is while disowning can't be reversed eviction can be reversed.
My best wishes to you and your family.