So can you please advise that how strong their arguments be proven in the court of law and what should we do to get rid out of all these i mean can we get Police clearance easily? Since no case is criminal till date and No FIR issued in our names till now.
Actually you have mingled two subjects together and asking for a single solution.
As far as your mother in law's inherited property from her mother is concerned, it shall become her own property over which nobody has any rights in it including your wife, however since there was a desire in the family to gift that property to your wife, you mother in law has transferred that property in your wife's name by executing a registered gift deed. upon this your wife becomes the absolute owner of this property, your brother in law cannot claim any right over it and any suit filed by him in this regard shall not be tenable or maintainable. So take it for granted that if the above circumstances are to be taken as true then he will be fighting a losing battle.
The criminal complaint by him, if it is based on the property mater, then you can inform the police that he has intentionally filed his to harass you all despite filing a civil suit hence this is not maintainable and whatever it may be you will take care of them through court. Since there is no notice or summons from police there is no case complaint pending before him so the police clearance for immigration purpose should be clear as of now.