Who actually owned the house before it was sold? If you were not a part owner, you do not owe any tax. If you were not an owner you didn't sell anything, so the money you received was not proceeds from selling the house. It was a gift from your brother (or from your mother). You do not pay tax on a gift that you receive.
It's not clear exactly what you mean by "My brother sold our mom's house." Was your mother still alive when the house was sold? If so, does "our mom's house" mean that she owned it, or just that she lived in it? If she did not own it, who did own it? If your mother owned the house but she passed away before it was sold, who inherited the house?
Is your name on any of the papers or documents from the sale?
