My mother comes from a family of assimilated Jews. Her great grandmother, though Jewish according to the Halacha, is buried in a Christian cemetery next to her non-Jewish husband (no crosses or Christian symbols on her tombstone). Same thing with her grandmother. Her mother lives and can attest to her Jewish origin. In addition, my mother has documents that prove her Jewish lineage all the way up to the early 17th century. Are my grandmother and mother Jewish? Are the proofs they have enough or does my mother need a witness that knew her great-grandmother?
The proofs are enough, as long as none of the woman converted to Christianity. The fact of the burial bears no relevance as long as she stayed Jewish. Rabbi Yosef Weitzen