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).
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.