Nice! In case anyone is interested in the science of this effect and it's surprising power, this is an almost exact reproduction of an experiment, in the field of "connection science" or "social physics".
(NOTE: The true value here is in the CROSS-INCENTIVIZATION with an existing social relationship, not just the money)
The general truism that falls out of it is:
When possible, always apply incentives to the social fabric, not individuals. i.e. incentivize the edges of the graph (the relationships), rather than the nodes (the people) in isolation. Seek results by always opting to strengthen social bonds, instead of trying to bend people (who are social creatures) into actions while neglecting their social context, which often pull them back into prior habits or prevent the new ones from sticking strongly. Under some experimental conditions, incentives are 30x more effective when you use the money to strengthen a social relationship around an activity, rather than incentivizing the activity itself. (Further, unlike a direct monetary incentive where removing the incentive strongly attenuates the effect, you could now remove the money incentive between you and your brother, and you're much more likely to keep performing the activity even without monetary incentive, because the persistent social incentive is bootstrapped.)
This and other really interesting related research was pioneered by MIT's Sandy Pentland. It's very solid and applies in practice: His lab won the DARPA network grand challenge, where they found those 10 balloons hidden across the USA. He wrote a book. Here's his author talk: https://youtu.be/HMBl0ttu-Ow
The book "Social Physics" is a really great read if the video speaks to you
Frankly, it's affected my worldview hugely. This sort of research has huge policy and regulatory implications imho.