Bluenose10 - What your neighbour is doing is wrong on every single level, and for the following reasons, I would report her:
1. She is being anti-social in ignoring the public health guidelines and government instruction. Those who behave anti-socially damage society. I don't believe that such people have the right to expect society's protection and take up a hospital bed should it be necessary.
2. If the visiting isolating family has access purely to your neighbour's house then yes, there is little risk to you but if those visitors have access to the garden then there IS a risk to you. The science shows that Covid19 aerosol droplets remain airborne for longer and travel further than originally believed. So an infected person who coughs or sneezes in your neighbour's garden sends a droplet cloud into the air that can travel over your fence and you don't even have to be there at that moment to be infected because the cloud will remain for minutes, so you might walk into it, or the droplets may land on your lawn mower. You are potentially being endangered.
3. You don't know that the visitors are self-isolating families, you don't know they're not visiting other members of their family or friends. If they are so cavalier as to make a visit to socialise and not just drop off provisions at the door, they are likely to be repeating that behaviour elsewhere. Our government failed to undertake contact tracing at the start of this problem and that's why we now have an epidemic in the UK.
Unfortunately, being in isolation and not watching the regular news and trying to keep all the nasty statistics out of mind, causes us all to become complacent, to forget that our hospitals have become warzones, and that our frontline workers are sacrificing their own lives for us. The least we can do is support them by yes - 'snitching' or whatever else people want to call it. We are in an existential crisis here, before long everybody will be asked to wear masks outside in contrast to the early advice. Personally, I would step up to the plate and not be afraid to do what we know instinctively is right here. It wouldn't matter that my neighbour would never be a 'friend' again because I don't want 'friends' who think they are above the agreed rules of society.