Assalaamu alaykum wa rahmatullaahi wa barakaatuhu. My parents are against me giving Da'wah (proselytization) and Islaah (reform), especially to my relatives through them (like the paternal uncle, maternal aunt) and to my father's work colleagues. They say that it puts people off and that if they complain to the boss that his son is annoying me, it will negatively affect my father's job; maybe they will even fire him. My parents are telling me these things. They also scold me if they find out that I did give these people islaah. By Islaah, I mean telling people that such and such is haram. Maybe my parents will take away my tablet (which I use especially to learn Islam), and mother sometimes asks if I sent any messages to my father's colleagues. I cannot lie. I do not know exactly what they will do if they find out, but they have given me bad threats, like making me stay in a hostel or in a relatives's house or taking away my electronic gadgets. I do not know if they will truly execute these actions. I have never disobeyed them so much. So, in my case, will I still get big sins if I do not tell them that Isbaal (wearing clothes lower than the ankles), smoking, putting their wive's photos on the internet, and music is haram? I am asking since I do not know what will happen if my parents find out. So I am stuck in a dilemma, sin and guilt on one side, and my parent's threats on the other. Please advise me in detail.

All perfect praise be to Allah, The Lord of the worlds. I testify that there is none worthy of worship except Allah and that Muhammad, sallallahu ‘alayhi wa sallam, is His slave and Messenger. 

There is no harm on you in refraining from giving Da'wah to these people and enjoining them to do good and forbidding them from evil in order to avoid making your parents angry. Enjoining good and forbidding evil is not an individual obligation required of you; rather, it is a collective duty that is obligatory on the whole Muslim community. An-Nawawi wrote:

“Enjoining good and forbidding evil is a collective obligation (Fardh Kifaayah) upon Muslims whereby if it is properly carried out by a sufficient number of people, then the objective is met and the obligation is lifted from the rest of the Muslim community. However, if it is neglected, then all Muslims who are able to carry out that obligation but neglected it without a valid excuse or fear of incurring considerable harm are sinful. There are cases in which an individual is obliged to enjoin good and forbid evil, such as if he is the only one who knows about the evil or that no one else can remove the evil except him.” [Sharh Saheeh Muslim]

Allah knows best.