I said "look dd, I'm going to draw the line here, a you should have saved some money for gifts and b I have no intention of buying a present for the person who went off with my husband!
She was only a baby when it happened - there is no way you can expect her to understand, not until she's much older, and certainly not at 11, and age when the world revolves around themselves!
To her, her dad's girlfriend is just that, and not the teenager that he ran off with - that's who she is to you.
You sound like such a loving mum, especially having been through such a rough time, and your DD does sound quite insecure regarding her dad and his girlfriend. Instead of getting frustrated with her, try to keep it positive; let her know that you love her unconditionally, and just keep in mind what a tough age it is, especially for girls, so try to take things with a pinch of salt.
It's really difficult, but you really need to try to keep your feelings about your exH out of it, because he is still her dad. I'm sure your DH has been absolutely fab, but because her dad is still in her life, he isn't a replacement in her eyes. It's frustrating for you to have to see, especially if your exH isn't a very good parent, but a rubbish dad will usually trump a brilliant step dad at her age.
Just sending lots of support because it's a tough situation to be in, and I think you're doing brilliantly. Apologies if anything I said came across as harsh, that certainly wasn't my intention.