Is the questioner sinful for giving some of the dresses and clothes she used to wear before her commitment and wearing the hijab to her unveiled sisters, and must she retrieve them if she is sinful?

The means are subject to the same ruling as their ends, and whatever leads to the unlawful is considered unlawful. Selling grapes to someone who will press them into wine is unlawful, because it is aiding in sin, based on the Almighty's saying: "And do not cooperate in sin and aggression." If the clothes given uncovered the 'awrah or had an unlawful aspect, and you knew that they would use them in an unlawful manner, then you have erred, and repentance is obligatory. Complete repentance includes retrieving them if possible, or obligating them not to display themselves immodestly while wearing them.