Am I sinful if my sisters sometimes use the internet for harmful things, knowing that I am the one who taught them how to use it?

The basic ruling for learning and teaching about the internet is permissibility, unless it is highly probable that the learner will use it for what Allah has forbidden. In that case, teaching it becomes impermissible, as it falls under aiding in sin and aggression.

Therefore, if you predominantly believe that your sisters will use the internet for forbidden things, you must repent and try to convince them to use the internet for beneficial purposes, such as useful research and Islamic websites.

However, if you do not predominantly believe that they will use it for forbidden things, then there is no sin upon you for teaching them.