Unless you two have a good relationship, it is best not to talk directly to her. Depending on the case, you can always talk to your husband and give your opinion on the subject. If he likes it, he will convey it to her as if it were his idea - she might listen.
It is not funny, it is not easy, and it takes years off a woman when she deals with her better half's offspring, but sometimes you just have to take a step back for your own sake.
As for your husband, tell him never ever to contradict or tell you off in front of anyone let alone the kid. Compromise saying that you will refrain from talking to her when you are not in the conversation, but you will still give your two cents to him - specially if it refers to family finances (in that case, you must be involved, heck with both of them).
Stepmom of one.