No one can sell your property without your signature. Contact your county recorder to confirm that the deed vesting title in you and your three co-owners is on record. If it is, then you can obtain a copy from them for a very small fee. If the deed is on record, then this realtor, broker and title company have a problem on their hands. If they are not taking your calls, perhaps it's time that they received a letter from your attorney.
I agree with getting a copy of the Grant Deed that shows you are an owner. Sounds like this may be an inheritance? Has the property been probated and distributed? or is it still in Mom's name.
The mortgage will transfer to the heirs as long as you keep the payments current. You can buy the property from the other 3 on title. They can force you to sell the property if you do not buy, but they will have to get a court order of partition. The listing your brother signed is only an employment contract between your brother and the broker.
The broker only gets paid by your brother if you agree to sell. He, the broker, does not have to speak to you and he knows he is in trouble or your brother misrepresented his interest in the property to the broker. You could write a letter to the broker telling him your side of the story....so far he has only heard from others. Do all of the others want to sell? and you are odd person out? If you can buy them out? or are you trying to make them keep their share, too?