How can I get a copy of my mother's Will?

I am so sorry for your loss.  Technically your brother is not yet the executor until such time as the court appoints him.  But some people let things go to their heads.  He should not give you the original and maybe that is all he has at this point.  But he can make you a copy.  You will, though, receive a copy formally when he submits the Will for Probate and to be appointed as executor.  If you do not get a copy then you should say something - or have an attorney write a letter.  Sign nothing unless an attorney reviews it if you feel some thing is not right.  And ask the attorney about time frames to contest appointment of your brother or the Will.  Good luck.