Am I sinful for my brother and father using the computer I will buy to watch songs and movies, and what are the consequences for me if I do not object to them? And am I sinful if I download a music player program for the Quran and they use it for songs and I do not advise them?

You must prevent the use of your property, such as a device, for unlawful purposes. It is not permissible for you to allow your father, brother, or anyone else to use it for unlawful purposes. If you do so, despite your ability to prevent it, then you are a partner with them in sin, an instigator of evil, and an enabler of it. This is based on the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." And based on the Prophet's (peace be upon him) saying: "Whoever guides to a good deed will have a reward like that of its doer," and similarly, whoever guides to an evil deed will bear a sin like that of its doer.

We advise you not to purchase this device if you cannot control it and prevent its use for unlawful purposes, for warding off harm takes precedence over bringing about benefit.