Hi there ,
Welcome to Just answer !
Well, there is no specific strategy that you may adopt while dealing with your son in law , as it is the hallmark feature of a person who goes in mania ( bipolar disorder) that he / she becomes extremely suspicious and delusional about people including their near and dear ones and loose insight into their own illness. And the same thing happens with him and so he under the influence of mania becomes delusional and starts blaming you for bizarre incidents .
So, the only thing you shall keep in mnd while going there is to safely keep away from him and try not to argue with him or reason out with him for his actions and just play down any altercation or incident that comes up with him on phone or in person and most importantly be in constant touch with your daughter on phone and let her know of the developments as your stay progresses and call emergency 911 , if you feel threatened and unsafe by his actions / attitude..
Besides , these precautionary measures , you may not be able to do much , so just do the needful.
I hope this helps..
Wish you all the best...
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