Is it considered a sin upon me to provide my brother with an extension to increase internet speed, knowing that he might use it to commit forbidden acts online, even though I benefit from this increase and the internet was available before that?

There is no sin upon you for providing a modem connection as long as you do not know the intention of the person taking it, and you do not know that they habitually use it for unlawful purposes. This is because the internet contains both benefits and harms, and it is permissible to lend the connection unless you strongly suspect that the internet will be used for disobedience. However, if you know their bad intention or strongly suspect their use of it for forbidden acts, then you would be sinful for assisting them in that, and lending it would not be permissible and would be forbidden, in accordance with the Almighty's saying: "And cooperate in righteousness and piety, but do not cooperate in sin and aggression." If you later learn that they are using it for sin, you must prevent them as much as possible, as a way of changing what is wrong, but without spying, for the Almighty's saying: "And do not spy."