How can I command good and forbid evil regarding my father's actions and behaviors, and also with those older than me, without showing a lack of respect and manners?

Enjoining good and forbidding evil is one of the greatest obligations, and it has been made a sign of the goodness of the Ummah. To address the challenge when fulfilling this duty with parents and elders, one must possess knowledge, gentleness, and patience. One should choose the appropriate time to speak, seek help from righteous people, and ensure that the father perceives your sincerity and concern for him. One should recall the address of Ibrahim (peace be upon him) to his father as an example of gentleness and patience.

As for the group you lead, the matter is easier due to the closer age proximity, and your practical example will have more impact than direct speech. To avoid conflict with their parents’ guidance, focus on teaching what is right, instilling fear of disobedience, fostering love for obedience, and cultivating love for the Sunnah of the Prophet and aversion to innovations.

Your presence in the group should be accompanied by benefit and reform. If this is not the case, or if it leads you to error, then you must leave the place, as Allah Almighty says: (And He has already revealed to you in the Book that when you hear the verses of Allah being denied and ridiculed, do not sit with them until they enter into another conversation. Indeed, in that case, you would be like them. Indeed, Allah will gather the hypocrites and disbelievers in Hell all together) [An-Nisa’/140].