First, I want to say I fully understand. I grew up with a half sister. Our mother was the "step mother", but I want you to know that it takes more then biology to be a mom. You have been there for your oldest all the time. Every skinned knee, every loose tooth. You are Mom, don't let anyone tell you different, not even your oldest or husband. To this day my sister calls our mom and not her mother whenever she needs help.
Your husband's ex should only have a relationship with your oldest daughter, your younger two of course will know who she is, but that should be about it. You can and should put your foot down on that.
As for your relationship between you and your oldest, take her out on dates. Talk to her, don't trash her mother, if possible try and say positive things about her, if you can't talk about yourself. Let your daughter get to know you, get her to understand who you are, have fun with her and with luck if the EX is saying things, your oldest might even stick up for you and even come talk to you about it. You can even have girl nights where you and all three do something together, help bond all the girls as sisters. You might also take your younger two on individual dates too, to keep things fair and it couldn't hurt to bond closer with them too, the teen years will come.
Try your hardest to not even think about the ex or even talk about her, think of her like you would one of your daughter's teachers or babysitter. A part of your life, but not really. Do not giver her power over your marriage.
Good luck.