Welcome ateograciasadios, I'm sorry that your mother's cult persona is trying to emotionally coerce you to behaving as a JW.
I would write your mother the following note to politely confront her and to find out what she is implying, because she did not write that she will be shunning you:
Dear Mom,
I'm very confused by your last letter to me where you wrote, "I will have to arrive to the inevitable conclusion that you have left Him, which means you are also leaving me." Mom, I love you unconditionally and have no plans leaving you. Are you planning on leaving me? What are you trying to tell me? Also, who is brother Brown?
I will call you (insert a time).
I'm (insert what you are doing or what you want to write her).
Love,
Pease be with you and everyone, who you love,
Robert