What is the way to sincere repentance and to get rid of recurring sins such as incest, sodomy, and masturbation, especially with a history of psychological and environmental difficulties?

These actions are major sins and immoralities that incur Allah's wrath. You must completely distance yourself from this relative and sever all ties with her. If you are forced to meet her, be careful not to be alone with her or see any of her adornments. Lack of steadfastness in repentance is often due to negligence in performing duties or persistence in forbidden acts. Do not let relapsing into sin prevent you from initiating repentance again, even if it happens repeatedly. Repentance must be sincere, not merely words uttered by the tongue. Increase your remembrance of Allah and perform both the obligatory and supererogatory prayers, for indeed prayer prohibits immorality and wrongdoing. Your relatives who are engrossed in sin have a right to your advice and admonition, but if they insist on their ways, then it is incumbent upon you to forsake them.