AIBU

I totally agree with Monica, Susan56, Quercus, Chewbacca and others who write in a similar vein. If the neighbour's actions are not directly harming you then leave them to it.
What sort of a society have we become that we are driven to pry and spy on our neighbours and other people? Their actions will sadly come home to them without your "help".
Who would you report them to? The Police?
Just take a look at the video of an officer in the Lancs. Police Force and ask yourself if this is what you want.
Far more worrying to me is the lack of social distancing in shops and supermarkets. Yesterday, when out shopping for my neighbours in my local supermarket, one young woman with 2 kids in tow thought it quite ok to lean almost on top of me to reach for something on the shelf above me! What could I do about that? Nothing, except remind her very loudly to keep her distance. Her response...a silly grin. Now that sort of idiocy is certain to cause the spread of this dreadful virus, more so, probably than your neighbours visiting their family members and yes, you don't know the circumstances behind those visits. So please, let's get a bit of common sense here. Bluenose, if your neighbour's visitors haven't jumped in your garden or invaded your house you probably aren't likely to be at much risk of catching this virus. I wish I could say the same for myself after yesterday's incident!