Regarding: "You mean it is up to the local bishop to decide what is sinful for the Catholics in his diocese and what is not sinful? "
- Historically that would be the case. For instance, one can imagine that in some local churches a bishop could forbid an act which if effected would incur at least the sin of disobedience to the bishop.
- On the whole, however,  the pastoral issue directly or in directly referenced in the foregoing discussions is not who sins and who has no sin, but what to do when or even if there is sin.