What is the ruling on using hashtags if they are used for something unlawful, and am I sinful if someone accesses unlawful content through my tweet?

Preparing hashtags falls under the category of permissible activities (mubah), and it is allowed as long as it does not lead to something forbidden. It is not prohibited as a pre-emptive measure (sadd al-dhara'i') if its benefit is preponderant, as is the case with grape cultivation. However, its preparation is forbidden if it includes anything unlawful.