Work issues. Am I ok for being upset?

I'll start by saying I work in a small medical clinic. There's 2 of us girls, the NP who is there only once a week, and my boss who is more like a mom in all the good and bad ways. My coworker, welll..  she's not somebody I would call a friend or even hang around if I just met her elsewhere. I'm a very calm, relaxed, understanding, worried about other people's feelings kind of person and she knows this. Makes comments about it all the time. She's EXTREMELY self-centered, dramatic, bold, and opinionated. It took me 6 months to finally get use to her. She's been at this place of work for five years. I've been there a year and a half. She's salary getting paid 40 hours a week but, only works 35 most weeks. She makes nearly $700 more a month than I do. We're open Mon-Fri. She has every other Friday off and only works til noon the other Fridays. She then leaves work an hour or two early twice a week. Her "alarms haven't gone off" three times in a month causing her to show up two hours late to work. She takes money from our cash box and pays her babysitter then runs to the ATM and fills the cash box back up with the money she took. She's constantly looking for ways to get out of work. She constantly tells patients she will fax labs to their doctors, order additional testing, so forth and never does it so, I hear the complaints. I FINALLY worked up the courage to say something to my boss and she has yet to say anything even though this was over a month ago. I can tolerate her personality. I realize personal life and work are two different things. But, what is getting to me so bad is her work ethic. Like why is it so hard to be at work on time, actually work, do what you're suppose to do and leave when you're suppose to leave. Everything falls back on me. She takes advantage of my boss because she is more like a mother and doesn't stand up to her. My boss has told me herself she won't say anything cause my coworker gets defensive so easily. She gets mad when she's late and leaves early and brings her outside drama to work yet still won't say a word to her. I look at it in a business standpoint though. I haven't said anything to my coworker about my feelings because I don't feel as though it's my place.  I just feel like if it were me, I would've been fired a long time ago. I just don't feel like all this would fly at any other job. She has it sooo easy where we are and she acts like it's so difficult. She blames having three kids. Well, nearly everyone has kids and works a normal job? We do the SAME job. Nothing is different between us but, she gets this special treatment. I just feel myself getting steadily more aggravated and becoming very resentful toward her.  Am I wrong and petty for letting this get to me or do I need to be told to sit the fuck down?