Twofish, I'm so cross on your behalf! The way your daughter behaved at the mall would shame a 2 year old and taking back that shirt was the act of a hero.
Of course you know what's going on, and are just building up the strength to resist it. I wish your husband would stand up for you more; I don't know if he heard the daughters disrespecting you, but if he did he should not have tolerated it.
It makes sense that while they are treating you this way you withdraw all non-essential help. I don't know if you're cooking, washing etc for them, but they don't deserve any of it. Otherwise you're just an unpaid servant.
Your son sounds like a perceptive person. Without heaping everything onto him, it might pay to tell him the plain truth when he asks what's wrong as he probably guesses anyway.