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That's good if you can maintain that, but who knows what will happen in the future. Local shopkeepers become infected and shop closes, etc. I suppose you could then choose to start stricter isolation at the point where any of you have to mix more.
We're in a kind of similar situation as we're in a house that's converted into 3 flats. We're keeping our distance from the occupants of the other two flats but downstairs have two kids who are about the same age as my daughter, plus they have a new puppy. It's driving both sets of kids a bit nuts that they can't play together but we're sticking to it, especially as the adults downstairs seem to be less strict about their social distancing.
Also depends on where/when each of you mix outside of the properties.
For example if you, in a couple of weeks, went to Shop/Supermarket A and bought some things.
Then, a bit later, if one of your in-laws went to Shop B.
A bit later someone from the third property went to Shop C.
Without the mixing there would be no connection (via your family at least) from Shop A to shop B or shop C but by mixing families you now increase the exposure/mixing risk for everyone else. People who also shopped in Shop C who never went to Shop A now have a possible connection (however small).
By keeping separate you help minimise this risk.