This little girl would probably want blonde hair like Mommy’s even if she didn’t have a blonde doll, too. That is OK. There are few brunettes who have never wondered what they’d look like with blonde hair. It is better to help her appreciate what she has than to play down what you have.
I had a friend whose mother would not let her have Barbies, because Barbie was not realistic. This friend grew up to have a tiny waist, DD+ bra, huge wavy mid-back length hair, impossibly long legs, and–I am not making this up!–she could put both ankles behind her head at the same time. IOW, this woman’s mother kept Barbie out of the hands of one of the 0.05% of women who would actually grow up to remotely resemble that doll.
Who knew?!?
The irony of the situation so amused this woman that one year in college she dressed up as Barbie for Halloween, which is to say she added a skirt and a furry short-sleeved bolero jacket and heels to a silver metallic one-piece bathing suit to make an evening ensemble. Yes, it was as silly-looking as it sounds, and no one had to guess who she was dressing to look like. Look at the clothes that Barbie comes with. The only way your daughter is going to grow up to dress like Barbie is if she makes it in Hollywood. In that case, she is probably going to be a lost cause on Oscar night, anyway.
As for Barbie, the main thing is to not let her brother get a hold of one. Immodesty is bad enough, but the acts of misogyny* that Barbie has endured from older brothers would curl your hair!

**
Seriously, though, I’d bring the girl up to see immodest fashions as sad attempts to be a piece of meat in order to gain attention, because that is what they are. That is the difference between modesty and immodesty, after all: immodest is the tendency to attract inordinate attention, not this amount of skin that is covered or not.
Add an ordinate degree of self-respect and the knowledge that nothing can replace “
the hidden character of the heart, expressed in the imperishable beauty of a gentle and calm disposition, which is precious in the sight of God” (1 Pet 3:4), and she’ll know what to pursue and how she wants to be attractive, both inside and out.
But yes, give Barbie some decent clothes. I always enjoyed making clothes for my Barbie. Buy thin fabric, though. You can’t make flattering clothing out of cloth that is half as thick as your wrist. This problem of cloth bulk is one of the reasons that naked Barbie has such odd proportions. When you add clothing, especially anything with a gather at the waist, and the other eye-tricking that happens when you scale a person down, the proportions aren’t so far off as they seem when you assess the naked doll with a tape measure.*