Solar City was a Musk (nee Rive) family business from the start.
https://en.wikipedia.org/wiki/SolarCity#History
> SolarCity was founded in 2006 by brothers Peter and Lyndon Rive, based on a suggestion for a solar company concept by their cousin, Elon Musk, who is the chairman and helped start the company.
https://en.wikipedia.org/wiki/Lyndon_Rive
> Rive is a cousin of SolarCity investor and entrepreneur Elon Musk, as their mothers are twins
(That means that Rive and Musk are genetically half-siblings.)
Apropos of nothing, I'm now imagining a world with forms of address based on the natural logarithm of shared genetic material.
https://www.npr.org/sections/codeswitch/2018/02/09/583987261...
I think the cousin thing is just explaining why they were interacting at all.