Trying to let it go

Don't worry, I'm not about to bust into some Frozen tunes, but I'm finding myself challenged at work currently.  Don't get me wrong, I love my job, like most of my coworkers and have a great boss.  I'm happier here than I've ever been at a job.  Most days are great, but right now I'm annoyed beyond belief and need to figure it out.

I work in a department of 4 women, one man and a male boss.  Most of the time it's harmonious and pretty laid back.  Right to the point of sometimes punctuality goes right out the window.  Last week we were pretty bad for it.  The boss will randomly come in late, and we'll all start to follow that pattern.  We all started to abuse the flexibility last week to the point where the boss sent an email about respecting the start times, but it only went to the ladies.  The other male coworker is one of the worst offenders, but that is actually a different story.  

This happens, it's cyclical, we get told to straighten up, communicate with the team better and obey the rules and we usually do.  This time, however, TCW is still running rogue.  We've been told to honour the start times.  There are two approved start times, either 8-430 or 830-5, and TCW continues to stroll in around 915.  She says she works until 6 but I know from personal experience, that when the boss leaves (usually by 5) she's out the door minutes later.  We're also supposed to let the team know what's up. She never does.  She wasn't in the office at all yesterday, and today I found out it was due to an "off site meeting".  She took an ENTIRE day off for a 1 hour meeting with a 90 minute drive on either side of it.  Then today, she strolls in after 915 again.

I know it shouldn't be my business, but she's been promoted (to a level above me) during my employment there and she's also had her workload reduced, where I keep getting more dumped on me.  

I know that life isn't fair, but it's not equitable and it's starting to really piss me off.  I could go to my boss, but I don't know how to voice my concerns without sounding like a whiner.  I could go to HR, but that's TCWs style, not mine.  I know that I should just let it roll off my back and let Karma get her, but I'm having a hard time letting this one go.