Is one sinful for following religious and beautiful accounts on Twitter that, at the same time, post forbidden tweets, and is one sinful if others view those forbidden things through his list of followers?

You bear no sin for following the owners of impermissible tweets if there is a legitimate benefit and you do not aid or participate in the sin. You must lower your gaze from forbidden sights and denounce evil with a good approach. The sin of intentionally viewing forbidden tweets falls upon the one who views them, not upon you.