Why doesn't she talk to me much anymore?
I get the impression that you bother to ask your question because you care for her in more than a casual friendship sort of way. You did a great job of explaining a scenario of how she is more shy now. You didn't say how old you are now or the girl or her brother, or if "she" is his older or younger sister so therefore I cannot tell how old you were when "she" talked to you, nor approximately how old she is now. I am guessing you are a teenager or just beyond it, but not much, and I would also gamble that she is younger than you and your friend. Presuming that you and your friend are very close to the same age, if she were older than your friend, you might not even care as much. Most likely she is younger and it is part of our genetic makeup to consider her to be more of a possibility to share a relationship with. Regardless, she is growing into a more mature person just like you. She may be more cautious of "boys" now, totally different than when the three of you were playing around as kids. Then it was just kids playing; it didn't matter. There are any number of other possibilities why she does not talk to you very much now, and the only way you could totally know is for her to feel comfortable enough to tell you. That isn't likely to happen. She is more likely to tell her best girlfriend over telling you directly.
There could be many scenarios of what changed. Here are a couple: she might have liked you before (even young kids have bonds to others that when broken leave a sad feeling) and your separation from her and her brother might have been taken as a rejection to her. Because of this, she does not feel as comfortable with you now; why risk her attention on someone that may end up disappearing again. What her brother is willing to accept as a friend now may not be the same as what she is willing to accept. Another possibility is that one of you has changed more than the other. For example, you may have tarnished your own reputation in some fashion, and you may not even realize it, or know that she knows. On the other hand, you may have went in the opposite direction, and she may feel like she wouldn't measure up to your level now, and as a result, she is not going to let you into her world just to be played with. If she has a girlfriend that you also know, if and when you become close enough as a group to talk to her girlfriend, and you know that exposing your feelings to her about her friend would be taken seriously, sincerely, and this friend would have the both of you in her best interest, then you might share some of the feeling you have about this girl. Girls like knowing someone cared about them, and they like the confidence of a girlfriend that agrees that the guy would be a good choice.
You might find that in the end, it works out fine with everyone. It is up to you to be on your best behavior when you are around her and her family. By doing this, if there ever were good feelings, there is a much better chance they will bloom again. Keep open, in your actions let her know that you can be trusted and that she is more special than most other ladies that you know. You don't have to say anything; just let your actions speak for themselves.
Should she develop a relationship with a different boy, then keep in mind you started out as friends and you should not change that just because there is some other person in her life. She will eventually know that you have her best interests at heart, and she may even depend on you to confide in about her other relationships. In that way, you might find that you two will have more fun times together than many other relationships. Sometimes a crush on a friend is all it takes to bind the two together for a lifetime as "just friends." Those relationships are many times much more important when writing your life's memoirs than a high-school relationship that only lasted until one went to college. Whatever the outcome, do not worry that an old friend has changed. There will be many other friendships that change, some become closer, some more distant. Sometimes it can't be helped; that is just how life is.