According to Halacha or Talmud, who takes care of a widow, whose husband had no brothers, and who herself has no brothers and no sons and no daughters?
As sabbahillel pointed out, everyone has a heir besides for a convert, so there would always be some distant relative that would inherit the estate. The Rambam states (ishus 18:1-3) that a widow lives off the the assets of the heirs until she gets the kesubah (prenuptial agreement). They have no right to force her to get her kesubah in order that they shouldn't need to provide for her, she gets food as long as she wants to, provided that she does not ask for her kesubah (magid mishna ibid). The Rambam adds that she stays in the house she used to live in when her husband was alive and uses all the objects in the house and even uses the slaves of the house as she used to before.