The question of prejudicial interests is covered by the Code of Conduct. The 'test' as to when a personal interest becomes a prejudicial one is whether a member of the public aware of all the facts might conclude that the interest will prejudice the member's judgement of the public interest. Really it is for the member concerned to determine when s/he ought to declare an interest. Your clerk ought to be able to advise. Specific advice can also be obtained from the legal dept. of your local authority.
Usually, issues such as these are indicative of a general breakdown in trust among members of the council. When the Code of Conduct was first introduced, it was an offence for any councillor who was aware of an undisclosed interest on the part of another councillor, not to report it to the local standards committee. This so-called whistle blowing clause was later dropped. However any councillor may still report any suspectedbreach of the code. This perhaps should only be considered as a last resort after all efforts to resolve the issue internally have failed.