Would a person be sinful if they paid for a service that resulted in viewing forbidden things after being advised not to watch them, and would they be considered an infidel if disbelief was viewed after paying the bill?

Paying the internet bill or enabling your brother to watch forbidden things on the device does not necessitate disbelief (kufr). However, whoever knows that the device will be used for forbidden things, then enabling them to do so is assisting in sin. Advice is not sufficient if it is known that the advised person will not cease their forbidden act. If your brothers watch forbidden things on your device, it is not permissible for you to enable them to do so, while you are able to prevent them, because the Prophet, peace and blessings be upon him, said: "Whoever among you sees an evil, let him change it with his hand; if he is unable, then with his tongue; if he is unable, then with his heart—and that is the weakest of faith."