What are a co-owner’s rights if they do not want to sell?

Under most circumstances, co-owners of real estate can freely sell their ownership interests (there are however possible limitations on inheritance rights). Additionally, if co-owners can't agree as to whether or not to sell jointly held property, the party in favor of a sale can file for a "partition" action.  This is a legal remedy available in such a situation as this.  Partition allows for the division of property among co-owners.  Accordingly, if a property can be physically divided the Court will so instruct.  Possibly since this is a farm, you could get 1/4 of the land.  However, if division would be impracticable, a Court would order a sale in lieu of partition and an equitable division of the proceeds among the co-owners.  Before doing so however, the court would permit one co-owner to purchase the interest of the remaining co-owner at fair market value.

1) As a general rule, any person may sell his or her share of property without the consent of other co-owners; the exception would be if there was some agreement or document which bound the co-owners and precluded this. So yes, your siblings may sell to the sister, and she is not obligated to offer them a "fair market price"--she can offer whatever she wants and they can accept it or not as they choose.

2) You should consult with a real estate attorney, but in a situation like you describe, yes--your sister would be able to sell, and you would be simply be entitled to receive your share, based upon your ownership interest, of the price.