J.J.
#1) You are lovely, T., not a fat cow :)
#2) Ex-Wife is very scared, hurt, and terrified of losing more in her life (lost Aaron, is now losing dd...looks like she is losing control of her own life, too. Must be crushing to feel like such a failure and knowing that it must be her own fault. That is a lot of guilt to live with and as a survival instinct she has to find a villain--or in this case, villainess, to pin all of this misfortune on).
#3) 11 year old is old enough to see the writing on the wall and to know who the "good guys" are without anyone telling her. It is hard to let her hear lies and slanderous things from her mom, but you and Aaron need to show support, not rivalry. When mom says something evil, you both need to agree to respond with a neutral "Your mom must have a lot to worry about right now and her feelings have nothing to do with how wonderful you are" kind of thing.
#4) Get the whole family into some very low key counseling. Just a safe place to chit chat and to have organized discussions...you don't need to be psychoanalyzed, diagnosed or medicated or anything...just all be able to talk as a family with no distractions for one hour a week and let your sd have a third party to air her grievances with. As much as she may love you all, she is growing into a woman now, too, who has private thoughts and feelings she cannot share with everyone. Be sure she has a journal to write in, and leave it alone no matter how curious you are to see inside of her mind.
#5) I recommended this to another mom of a teen girl, but get the book Reviving Ophelia...this is a precious time you have with your 11 year old...this time can really shape her whole future as a woman and if she is to come out of the other side as her authentic self then the work has to be done now. We should get together so she can hang with my 13 year old (you know who) who is an example of how girls can get through a bad relationship with one parent and develop a loving, whole relationship with her step parent.
#6) Stop making yourself sick. you have your new little baby growing inside of you and he deserves to have a loving, stress free womb to thrive in. (I have a great movie that illustrates how the womb environment lays the groundwork for your babies life....What Babies Want) Let this woman and her agenda go from your life. When her presence invades your life,mind or soul, tell yourself how awful it must be to be HER, and count your own blessings (literally) as a meditation to calm yourself down and to connect with your growing son. (Breathe in, i am blessed because i have my health, breathe out. Breathe in, i am blessed b/c i have love for myself. etc...) This woman got to where she is out of self loathing and bad choices, you possess the power to learn from her mistake and make healthier choices for your family.
I wish you light and love, T....get in touch with me or KDB so we can give you some support outside of the cyberworld :)