What is the best way to prevent potential issues of conflicts of interest?

The best way to deal with potential Conflicts of Interest is to openly declare these. Just because you

have a relationship with someone doesn't mean this is going to be an issue or lead to fraud, but it is a

good idea to keep a written record of these.

It wouldn't be a risk register, because how can you rate someone's risk of doing something dodgy?

Risk registers are for objective risks such as Health and Safety.