My granfather build a house about 60yrs ago on a plot that was surpose to be my gran's. Its a doudle plot with two house?after years we found out that her sister sons "iherit" both plots.each house was surpose to go to my gran and sister's eldest son.now he wanthe plot and the house, the house which my granfather build.
I can't say with any legal certainty how the law would look at this issue in Africa.
I suspect the answer will be similar to what I'm going to say, relative to US law.
What you have in this case is adverse possession.
If things are as you have related, your grandfather no longer owns the home, because of adverse possession.
This goes back to English Common Law.
If someone owns property and fails to protect that ownership PROPERLY for a lengthy period of time (10, 15, 20, or more years), their title passes to the person that occupies the premises adversely.
Hence, the title, adverse possession.
In your example, someone has officiated, occupied, inhabited, or used the property adverse to the interest of your grandfather for 30 years.
During this entire time, your grandfather (who unfortunately now lies deceased), did nothing to remove this trespasser.
Thanks to your grandfathers inactivity, the trespasser is now the owner.
Who says crime doesn't pay?
Anyway, in the US, the title (ownership) has passed to the trespasser or interloper.
Have you heard the expression, "possession is 9/10ths of the law"?
Yep, adverse possession is behind that maxim.
I suggest you consult with an attorney in the country on the African continent where this misdeed took place.
There might be other remedies under that nation's law.
Good luck!!!!