Will the sister be considered a single mother?

It will be considered. According to Article 18-1 of the Law "On State Assistance to Families with Children", if a single mother, a single adoptive parent, a mother (father) has registered a marriage in the event of the death of one of the parents, then they retain the right to receive assistance for the children who were born or were adopted before marriage, if such children were not adopted by the husband (wife).