You don't have to be afraid that how your brother would react after knowing that you have changed your surname. This is something which anyone can do. Well the very simple answer is that now you have realised that you love your mom more than your dad. Only if you have to tell anyone some sort of explaination otherwise what I believe that you are a bit afraid in advance. hopefully he won't ask you much or enquire much after knowing this and it would be fine if you reply "Doesn't it look good?"

There are some secrets which should remain secrets in life and no body can understand your position. its better not to share this thing with any one in the family what every has happend just try to cover it up in your past.

Best of luck