I am the mother of a nine-year-old girl and a six-month-old son.
Since the birth of my son, my daughter has asked relentlessly if she could have a boyfriend. I explained that she isn't old enough; she can have friends that are boys. I'm aware that many of her friends' parents allow their children to have "boyfriends" and "girlfriends." My husband is not her biological father, but she thinks of him as her father. Given that many of your daughter's friends' parents allow them to have boyfriends and girlfriends, it's pure conjecture to say that her six-month-old brother's birth has been the driving force behind her demanding to have a boyfriend.
Put yourself in her place and talk to her with empathy and understanding, not blame and disrespect.
As you continue to make her feel special and stay in touch with what's important in her world, you'll learn better how to counsel and understand her.
She may have felt displaced by him in your family, regardless of how attentive and sensitive you have been to her emotions.
While discussing this topic, work in some questions about how she's feeling about other aspects of her life--school, social life, extracurricular activities.Also ask her how life has changed for her since her brother was born.