So, you better not cheat unless you already know what you are doing.
The locked progress bar mechanic is in place so the game remains narratively consistent, I would guess. Once you get to it, an event is needed to progress further/unlock the bar until the next event stop point.
In case of Minami's siscon, or rather, Mike's (willingness to do his step sis), there's several events that are almost impossible to miss as they happen around the house and many more that just raise it rather than unlock it. There's only really two that you need to intentionally get out of the way to trigger. The "practice" date on Saturday (I believe there's a practice date icon at the house on Saturdays afternoon (13-15?), otherwise just invite her for a date).
Or, if you already got to a 100% once before, you need to take her on dates through various locations.