Property related

Without his relinquishment, you cannot transfer all the properties to your elder son.  As he has 20% in property, you can transfer the other 80% through a Gift deed.

If your younger son is the joint owner then you cannot force him to relinquish his rights in the property, he has to voluntarily do it.

Being absolute owner of the subject your younger son is solely entitled / authorised to transfer title (ownership)  his property.

Your daughter in-law has no right, claim, interest and title in the property owned by her husband / your younger son.

As the parent, your obligations is to assist couple to sort out their issues. Instead, why you want to overawe them?