This is my response to you:
1. You can call for the surveyour of the land and ask him to make the measurements of each person's share of the land;
2. After the demarcation has been done, then sell your share;
3. If your brother's still object then you have no option other than sending them a legal notice;
4. If they don't respond then you can sell your share to anybody because you have the right;
5. If they object then you have to approach court;
6. Also to comfort your brother's tell them that easementary rights will be with the control of the brother's therefore they need not worry.