Q: My father in law passed away 20 years ago and my mother in law remarried and her second husband also died. Is it permissable to bury my mother in law in the same qabar (grave) as my father in law? (my husband's father/my mother in law's first husband)
A. It is permissible to bury one Mayyit in the grave of another only if it is certain that the body of the first deceased has disintegrated and become sand. This is mentioned in most of the books of Fiqh. After 20 years it is possible that this has happened; hence it will be permissible to bury your mother-in-law in her husband’s grave.
And Allah knows best
Mufti Siraj Desai