Then, if she doesn't change, then it might not be a good thing to change her. People change. Maybe your friend in changing. People grow in and out of personalities. The chances are this one wont last forever. But while it does, you should accept her for who she is.
I don't know your reasons for not liking Brittany (and they seem irrelevant anyway - you don't have to get on with her), but you have to try explaining those to Sara as well.
Why fo you not get on with Brittany? Maybe there is a way of resolving that. You can't just not like someone - if you feel as though she has done something wrong, try resolving that with her. It's not Sara's fault that you and Brittany don't get on. It's worth a try for the sake of your friend.
Or talk to Sara, tell her you feel as though she is changing, and why you don't like Brittanty, etc.
Tell her - you don't have a problem that she's found new friends, it's the friends she's found. Either way, you need to talk to one or the other.
PM if you want to talk xx