1. Yes, you can sue the original owners and the estate agent and the solicitors who acted for you in the sale. There would be different causes of action against each person. As against the original owner, you would sue for misrepresentation and deceit. As against the estate agent, you would also sue for misrepresentation and negligent misstatement. As against both your own solicitor and the vendor's solicitor, you would sue for negligence and if the solicitor knew of the difficulty and concealed it, for fraudulent concealment. Be aware that there may be Statute of Limitation difficulties with certain of the causes of action, but not others. Any action based on deceit or fraudulent concealment is never statute barred. However actions based on negligence, misrepresentation and negligent misstatement have time limits of three years, so it may not be possible to successfully sue on these grounds.
2. Be aware additionally, that given the close family relationship between the vendor and his brother who owns the right of way, it may be possible to claim an easement of necessity. But this is a much weaker ground on which to have a right of way than to seek rectification of the transaction on an equitable ground such as deceit or fraudulent concealment.