In the mentioed scenario, a Muslim woman will bathe her (i.e. the deceased woman) and dress her in her Kafan (i.e. the white cloths that the deceased’s body is wrapped in). The expenses for this will be taken out from her (the deceased’s) own wealth. Then, since she does not have any Muslim relatives, her Christian brothers will be most deserving of placing her in her grave.
And Allah knows best.