Best friends have a relationship that can't be broken. If you are worried about the bond you two share, then it can't be that strong. Maybe time to add some new friends to your circle. You don't have to give up your current friend but don't close yourself off to new friends. Most people have many best friends as life evolves. People change. Yourself included.
You need to sit down with your friends and talk about what is happening. They probably have also noticed what this new kid is trying to do, so coming up with a plan or agreement should be easy. Alternatively, you can talk to this new kid and let him/her know that you see what s/he is doing and you won't stand by and watch.
I do not how a arrival of a new kid break relationships,cause your question is much like a riddle,still if your relationship break with your best friends,then they are not your best friend , Friendship is name of trust,helpfulness and love(not BF/gf kind off love).