You must not represent a client in a matter where there is a conflict of interest unless you have received express or implied consent to do so from all affected clients and you reasonably believe that you are able to represent the client without having a material adverse effect upon the representation of or loyalty to the client or another client.  Full and fair disclosure of all information which may be relevant to a person’s decision and in sufficient time for the person to make a genuine and independent decision is essential.  If you are not able to provide complete disclosure because of the confidential nature of the information, you must decline to act.
Also, while it is not required by the rules, in some cases you should recommend that your client obtain independent legal advice about the conflict of interest. A general open ended consent obtained in advance from your client may be ineffective because it is not reasonably likely that the client will have fully understood the material risks involved. 
See rule 3.4-2 of the Code of Conduct for additional information and also for information about when consent may be obtained in advance and under which limited conditions it may be implied.