1.Since the property was purchased in the joint names of two of you,the proeprty is a joint proeprty.
2.in that event unless you make a physical demarcation/partition of the same you can though sell but can not give the buyer possession of the sahre.
3. So make a written agreement with your BIL about rhd marcation and then sell your sahre.In the sale deed do make your BIL as confirming party so he may not oppose this sale alter.