Is it obligatory for a young man to boycott all of his relatives who commit immoral acts and do not accept advice, while maintaining communication with their children and offering them advice?

What you have discovered regarding behaviors within your family contradicts religion, morals, and sound human nature. The reason for this is their deviation from religion. It is not permissible for you to condone these behaviors or remain silent about them, otherwise, you would become a "dayyuth" (a cuckold, one who tolerates indecency among his womenfolk). The Prophet, peace and blessings be upon him, said: "Three people Allah has forbidden from entering Paradise: the habitual drunkard, the disobedient to parents, and the dayyuth who approves of indecency among his family."

Your duty is to forbid evil with your hand if possible. If you are unable to do so, then with your tongue. If you are unable to do so, then with your heart, and that is the weakest of faith. You must advise them, for "religion is sincerity/advice."

You may abandon them if you hope that it will have an effect on them. This is not considered breaking ties of kinship if done with this intention. You should invite the younger members of the family and those adults who respond to you (to the straight path), and be an example for them in your uprightness.