So this has been done in some of the custom galaxies I think, but my idea has 2 parts.
Depending on galaxy size and fam size, each family has a set number of systems that cannot be expoed by other families. These systems would be the closest to their home system. There is even a verson of this where these systems could not be attacked by other families. This would reduce farming someone into oblivion.
When making an nap each family gets to select X number of systems that will be a “border” and cannot be expoed by the other fam. The amount of systems (X) will likely change round to round as number 1 above. This will reduce the amount of DPas needed and prevent bad blood from “unintentionally” crossing borders.