Sisters are not to wear pants to meetings, assemblies or field service. Their dresses/skirts should be modest, not too short, not too tight, etc. Tops/blouses should not be too low cut or too tight. Sleeveless is okay for summer but only if modest and not exposing too much cleavage.
There are no written rules. The WT magazines are referenced to determine what a "Christian" would wear but no hard, fast rules are laid out. In a lot of conservative areas, the elders or older sisters will wield a lot of influence on the younger and newer sisters on what is acceptable or not. That's why you hear of some areas requiring sisters to wear pantyhose while in other areas it is never been an issue.
The same thing happens to brothers. Brothers are to wear a tie at all meetings, assemblies and field service. If giving a talk or part on the platform, a suit jacket is required. Some areas have tighter rules about what brothers can and can't do. In one old hall I went to a brother had to wear a suit on the platform...dress slacks and sport jacket were not allowed. In another hall, brothers were not allowed to wear waist length jackets in service...they had to be three quarter or full length overcoats. Some brothers in the Western US were not allowed to wear cowboy boots with suits. Those rules are not written in WTS publications but were enforced by local elders. Just depends on how much power the local elders have over the followers.