We live in a system with multiple rocky planets, but only Earth hosts intelligent life. How did Earth and Venus become twice as large as Mercury and Mars? Did Jupiter help or hinder the growth of the terrestrial planets? Multi-planet systems are natural laboratories that will help us answer these questions. I am leading a NASA Key Strategic Mission Support Survey to measure the composition diversity in a magnitude-limited sample of multi-planet systems. I will discuss my team’s recent mass measurements, including characterizations of rocky planets and discoveries of distant Jovian companions, and highlight future opportunities for characterizing rocky planets and their siblings.