***People learn how to behave by imitating other people. Thus the best way to teach your children to be "well mannered" (a vague term) is to be well mannered yourself. That being said, a child will imitate different people at different points in their cognitive development. I think it can safely be said that after parents, peers have the greatest effect on a persons behavior patterns. However, negative behaviors are often wrongly attributed to coming from peers, when in reality it is the parents who have had this effect.
***I will encourage you, as I do all people, to always ask "Why?". Examine your own upbringing and peer group, your siblings' upbringings and peer groups, and compare what you see to each individuals behavior patterns.
***Important things to remember in raising a well mannered child: (1) Practice what you preach. (2) If you think your child's peers may be having a negative impact on their behavior, do not forbid them from seeing these people. Rather, explain to them your thoughts, but under no circumstances be the least bit condemning. Condemnation of a behavior of of their peers will immediately shut the conversation down. (3) Avoid value words: one thing is not "good" and another thing "bad". To return to your earlier example: ask yourself why you told your little brother not to swear. What is wrong with swearing? If the best answer you have to this question is that its "bad" (or any synonym to the word "bad"), then you have no right to tell him not to swear. On the other hand, if you are able to coherently explain that swearing in most situations will cause people to look down on or ostracise you, then do so. (4) Do not ever say "Do not (do that). Rather, explain why, with the goal and intention making them WANT to do what you want them to do. Do not simply force your beliefs on them "for their own good".
<<<I would like to interject here and say how unlikely it is that your brother punched you in the face because you told him not to swear. I imagine you told him not to swear, and he first retorted verbally because you tried to tell him what to do. There is more to this story and it is important. No mentally stable person would punch someone because they said "Don't swear. Its vile". However they may be annoyed and for a very good reason: you broke rule #4 by telling them what to do, rule #3 by using a value word ("vile"), and rule #2 by condemning their actions by use of a value word. Just something to keep in mind.>>>
Rule number (5) is the golden rule. Before you say or do anything to anyone, put yourself in their position and ask how you would feel. This takes practice, but keep at it.
***On the subject of video games and being "spoiled" (by this I assume that you mean they are used to having their every whim granted), yes: violent video games can have this effect, but only if the player forgets its just a game. It is my personal belief that video games are a complete waste of time, unless they are being used for training simulation purposes. However this is a subject of personal preference and if you do choose to provide your children with video games, simply be sure to explain that it is a luxury and just a game. Just as you should explain the significance of everything you provide for your children.
Oh, and rule number (6): never resort to using guilt or shame as a weapon. This destroys people.