I don't think that your situation can be mended. You mentioned that you think your bosses already know what is going on but do not want to deal with it. If they have no backbone then they will be of no help to you. It sounds like they are afraid of them too. These two biddies are ruling the roost!
And if you report them and they are made to do more work then they will resent you even more. I think that you should look for a new job and leave this lot to stew in it.
That company do not deserve a hard-working and conscientious employee like you!
In the meantime can you be sly? The next time something is not done say " I thought you were doing that" or shrug your shoulders and say " not my problem" Try and show them up for what they are at every opportunity.
You are the office Cinderella however I would not advise waiting for Prince Charming to come along and rescue you ( you may have noticed that they are in short supply)Sadly I think that you will have to sort out the ugly sisters by yourself - with a little help from your friends on here of course!