@Shunna,
Your mother can sell her share of the house, she only has one third. Your brother can also sell his share. They are most likely not in and position to force you to sell your share and unless you are ruled in some way incompetent, they most likely cannot just sell your share without your permission. However, there may be other factors, perhaps a clause within the will, which indicates that if two of you decide to sell, the third must also sell. It will also likely vary according to where you live.
In other words, your best choice is to contact an attorney (solicitor), who will be able to better guide you on this matter.