Is it permissible to ostracize a sister who committed adultery and bore a child out of wedlock, then persisted in her actions by defrauding people and taking their money, after attempts were made to help her repent?

You should continue advising your sister and reminding her of Allah, the gravity of zina (unlawful sexual intercourse), and consuming people's wealth unjustly. You may boycott her if boycotting benefits her, otherwise, do not. Her guardian must be firm with her, confine her to the house, prevent her from going out, and warn people about her deception, as advice to the Muslims.