Relationship at break point - lazy relative issues

I'll try to keep it as short as possible. Started dating my gf 3.5 years ago. She moved into a new place with her 10 y/o daughter and 21 y/o brother 2 years ago. I moved in with them all last year. Here's the problem: There was a verbal agreement when I moved in that I pay 1/3 of the bills, makes sense with 3 adults in the house. Her brother let's just say is a very unmotivated and lazy individual. He was laid off from his job in December of 2015 and made little to no attempt to find another job and spent the first 5 months of last year not only not working but not doing anything to help around the house. He works now but still does next to nothing around the house except eat a lot of the food and leave a mess in the sink. To pick up his slack, my gf let him off the hook and now he is only paying $300/mo (I pay about $1200) which is slightly less than 1/3 of the rent and he pays for nothing else. His not working for months and high car insurance payments due to getting numerous speeding tickets have put him in a bad spot financially. My gf has absorbed most of his debts although I have paid a lot to help out with the food bill. I give her $700 on the 1st of the month to cover the 1/3rd of everything which includes my portion for food, but since we are constantly running out of food, I pay extra on top of that (about $400-500). Life in this house consists me running my gf and her kid to work and school every morning (she doesnt drive), even when I am coming off the graveyard shift, constantly cleaning up dishes, taking the garbage out every week, taking care of the two cats which aren't mine, cleaning the house in general, sometimes rushing home from work because she has company coming over, fixing stuff around around the house, including painting several rooms and spending 3 days painting her daughters room... etc. I could go on but you get the point. I do more than my share, often times picking up the slack and mess left by others. But in a recent argument I realized that this apparently comes with the territory. Her brother is what he is, I am just supposed to deal with it. And apparently when her brother wasn't working last year and she took on more of his debts she was hoping I would step up a pay more (although I wasn't directly told this). These issues have caused my mood to be affected which in turn is affecting our relationship. I struggle with dysthimia as it is although she doesn't seem to be too sensitive to it. When I told her that she would probably throw me out of the house if I did what he brother does (or doesn't do), I was told that I was f*cked in the brain. We tried to plan a trip for my 40th birthday in May but she backed out of and scheduled a retreat right after it so I guess we aren't doing anything special for my bday. I don't doubt she loves me but she does not seem to be taking my concerns too seriously. Yes I do value money, but it is only because I have a sick mother that lives far away that needs financial and I want to start saving for retirement, not enabling a soon to be 22 year old man. Plus, if we are going to have a serious future together, I do care about her finances and I hate to see her hand over a large portion of her money every month to a grown ass man. Is it wrong for me to see a problem here? I know I was the one who moved in with a crowd but why should I just accept paying extra (even if its a little extra) and working so much harder because others simply won't? This has severely affected my mood. I work 50 hours a week at my job and I just want a bit of a break when I get home. I know its normal to spend some time and money helping out but when cant everyone pitch in? I suggested counseling but her response is "I don't need help, I'm fine." I am at my wits end. What should I do or what would you do?