Let's all stand up together and do the "wave." You go girl. Glad to see that you're putting your foot down and letting your MIL know she's overstepped her bounds. It is absolutely unkind and unacceptable that she treats your H and Bree differently than the rest of your family. You are right to resent this behavior. It's hurtful to you and even more hurtful to your other children. You are right, you simply can't let this go on.
So, here's an idea to really drive home your point. How about you pack up Bree's laundry, hubby's laundry, your laundry, and your other two children's laundry in individual packs. Then place them in a place where she's sure to find them, with a note attached that reads, "Since you seem to enjoy doing laundry, here's an idea, why not do EVERYBODY's laundry." And oh yes, "No Starch, Please."
She'll get the hint loud and clear. You've tried being nice, but she doesn't seem to get the point. So, make your point very clear. Your MIL will likely be offended, but she'll respect you more when you start standing up for yourself. And little by little, she'll learn that there are consequences to her unkind actions.
Of course, if that doesn't work, you could always pat her on the back when she's on the way out the door, and clandestinely attach a stupid sign....