C.E.
YOU can't do anything - your HUSBAND however can. He needs to tell her that any and all communication will be with him and him alone. He also needs to explain to her - and not give an option - that you will and are in their lives and she needs to just get used to it and grow up.
And, (I'm a step-mom too, so I know of which I speak!) my earlier statement of you can't do anything was a little wrong. DO NOT acknowledge her letters or anything else. DO be there for the kids and be a mom (as much as you can) with them just as you are for your daughter. Make no difference between them. DO NOT disparage their mom in front of them or anywhere you may have any chance of being overheard.
Trust me, they will remember who did and said what as they grow up and they will love you for who you are and for loving them - also for NOT attacking their mom and making them feel like they have to choose. It sounds like she is probably doing a lot to make you look and sound bad to them - but, I promise, they will see the truth of the situation themselves as they grow up. Take the high road even when it's unbelieveably difficult!
Being a step-parent is absolutely the hardest thing you will ever be - especially when you love them and have a difficult ex to deal with.
Stay strong. It will pay off in the long run.
Good luck and God Bless-
C.