No real words of advice, but a big hug of support. I am also a fairly new-step mom. My dsd was 9 when I came on the scene and is 12 now. I have to say, she brings out the worst and the best in me. She only lives with us part time.
Be yourself, go with your gut, and be VERY expressive with your love for her. Also, with a new babe around this might be difficult, but it has helped my DH and I to make sure we take some time for ourselves when DSD is around.
Finally, communicate with your husband. This was the hardest thing for me to do b/c there were a lot of jealousy issues involved on my part and who wants to admit they are jealous or any other "bad" feelings. But DH was amazingly understanding and talking about it helped him see how I was feeling and helped me feel better.