Is it permissible to bury my cousin in my mother's grave?

It is forbidden to exhume the mother's grave to bury her cousin's son in it, unless it is confirmed that the mother's bones have decayed and turned to dust. This is because the grave is reserved for its occupant, and it is forbidden to exhume it before decay. However, if the body and bones have decayed and turned to dust, then it is permissible to exhume the grave and bury someone else in it.