Should a Muslim sister ask for the return of a guitar she gifted to her brother before she embraced Islam?

Your acceptance of Islam has brought solace to our hearts, and we ask Allah to keep you steadfast. Islam nullifies the sins committed before it, so your conversion to Islam has forgiven all that you did before Islam, including gifting the musical instrument to your brother. You are not required to ask for its return. You must strive for the salvation of your brother and your family from the Fire by advising them and inviting them to Islam.