@FredericConrotte A question would be how you could identify family members. Assuming everyone has their own devices, you might be able to use MAC addresses. But those are only visible within the network, so either the quotas have to be enforced by the Wifi access point itself, or the access point has to operate in bridge mode (with routing being performed afterwards). Another alternative might be a separate (virtual) network per user.

Instead of setting strict quotas, simply limiting the available bandwidth could be very effective. Even consumer routers have some QoS settings that could be useful here.

But given that your goal is CO2 reduction (not making life difficult), there might be more effective ways towards your goal. Technical measures should also consider the CO2 cost of producing and running extra hardware. E.g. streaming a movie can be better than pressing and shipping a single-use DVD. There is also opportunity cost: the money invested here could also be used to offset emissions another way.