The policy we seem to be developing over on Raspberry Pi stack exchange is one of relative permissiveness with regards to strict cross-site division.
Thus we have had a number of questions which should strictly have been moved to Electronics or Unix & Linux but have stayed on Raspberry Pi because while they could be answered on those other sites, they are still directly relevant to Raspberry Pi users and answers on
raspberrypi will be more appropriate to Raspberry Pi users than an answer on a sister site might end up.
In many ways, it's a shame that questions can't be shared between sites, as there is often genuine and unavoidable overlap between the scope of some sites.
Unfortunately the logistics of shared questions would be horrendous (since the moderation would have to be shared as well as the questions and answers) and these shared questions would almost certainly be abused, so I can understand why they have never been (and probably never will be) implemented. It is the Tragedy of the Commons sadly, so we all just have to route around the problem instead, hence discussions like this.
Anyway, what I would suggest is that you migrate questions which aren't actually about Sustainable Living but be relatively tolerant of questions which are, even if they would be on-topic on another site.
As a final point, I would strongly recommend that people read Shog9's blog post Respect the community – your own, and others’.