I don't have a relationship with my birth mom, and I've been living with my dad and my step mom for 3 years now. She has three kids of her own, 2 moved out, and one at a university. I am 19 years old and attending a local university, so this unfortunately means that I am living at home. When it's just me, it's not too bad, I do what is asked of me chore wise and most days, more than that. When my step brother comes home on college breaks, my chore load doesn't lighten as it should. Instead it gets heavier to accommodate cleaning up after my step brother. 9 times out of 10 I do the work without complaining, because I hate starting trouble. But a few months ago I mentioned something about the unfairness of the situation to my dad. Never. Again. He said something to my step mom and she FLIPPED OUT. She didn't talk to him OR me for 5 days. When they finally made up nothing was discussed, my dad just told me to move on to avoid drama. Most recently, yesterday I had plans to go out with friends at 2. Before I left I emptied the dishwasher, took out the trash, and cleaned the living room. I was especially proud of the living room because I vacuumed, dusted, and polished the tables and couldn't wait for them to see. When I got home, step mom was already home. I walked up the stairs and the first thing out of her mouth was a sarcastic "Thanks for doing the laundry." I just couldn't believe my ears. She didn't even notice everything I had done. I hadn't even realized there was laundry to be done. I apologized and went straight to bed, but it's still bothering me and I don't know what to do. Help?

I know this is easy for me to say, because in the moment we don't always say what might be best, but I will: an apology was too far in the martyr direction. Take a deep breath and tell her without anger that you put all your time and energy into the living room, and keep walking up the stairs.
The deeper problem still exists, of course. It isn't clear what her thinking is: you are not her daughter? You are female and she thinks men don't have to do housework? You live there full time and he does not, so you should do more? You live there and she resents that any child still is at home, regardless of bio or step?
Ideally you would find out. Try to find a quiet day and calmly ask her if any of the above apply. It might be the last two. It might be your exchange for room and board. Maybe it's time to find a cheap roommate situation.