First, she's NINE. YOU are the mother. If YOU feel it is inappropriate for a nine year old (and seriously, I don't think a nine year old needs to be wearing anything that makes them look bigger, older, whatever...) then she shouldn't be wearing it. Little girls should be allowed to be just that... little girls. Nine years old is STILL a little girl! You're not over reacting.
Second, in my humble opinion, it was inappropriate for the step-mom to buy that for her without first discussing it wiht you. Regardless of how well you get along. She's YOUR daughter.
I would have a conversation with your daughter. Tell her that you understand she does need to be wearing a bra, but you just don't think the one her step mom bought is the right one for her at this point. Tell her that you will plan a special day, just the two of you and go purchase a pretty bra that is just right for her. You could have lunch together, make it all very special. I think if you are just honest with her, try to explain your concerns in a way a nine year old can understand, but at the same time let her know you understand she needs it and want to help her with this ... then that will go a long way.
In addition, you DO need to have a converstation with the step mom. She needs to know she crossed the line. First, this is a right of passage that a daughter should go through with her MOTHER. Second, even if you were ok with her buying a bra for your daughter, she should have discussed it with you first to be sure you were ok with it and to find out what you felt was appropriate.
Finally, I think this is a bit of a red flag as far as what Dad and Step Mom think is ok for little girls. You may need to have a conversation with them on how you all are going to proceed into the next phase of your daughters life. How are the pre-teen and teen years going to look. What is ok to do, what is not ok to do. What is OK to wear, what is not. The LAST thing you want is for her to have certain rules at your house and then go to dads and be able to do something you aren't comfortable with. Like, I know I will never be buying my daughter belly shirts. I just don't believe it is appropriate for young girls to be wearing jeans slung down to there and shirts hiked up to here. If my husband and I were divorced, I would be sure to have a conversation with him to ensure we were on the same page. Consistency at any age is critical.
You didn't over react. You're watching out for your daughter.