The second marriage is not valid and null and void in the eyes of law especially when the previous marriage subsists and the spouse is living.
The children born to the second wife (concubine) are no doubt legitimate but they are not entitled to a share in the ancestral property of their father.
In your situation, in my opinion you say that your grandfather's property as ancestral property, if yes, then you are mistaken.
The grandfather's property is not ancestral. The share of property inherited by your father shall be his own and absolute property.
In that neither you nor anyone has any right or share as a right during the lifetime of your father. No court will come to your rescue because it is not your property, it is your father's absolute and own property and he can dispose it in any manner and to any one of his choice.
You may consult a local lawyer with all details and relevant papers/documents and take his advise on all such further issues.