The Kryptonian Kara Zor-El actually became a Legionnaire before her cousin Superboy. She joined at the same time as her soon to be romantic interest, Brainiac 5. She died fighting the Anti-Monitor during the Crisis on Infinite Earths.
The powerhouses, Mon-El, Supergirl, and Superboy, are all the same ability-wise with the added Time Travel (my take on Magic's Ninjutsu) for the super cousins. Power & Toughness differs as per the comics, from Mon-El (strongest) to Supergirl (middle) to Superboy (weakest).