Speaking as someone who isn't Chinese but has read a bunch of Chinese xianxia stuff so might get your confusion better, "little brother" is common Chinese slang for dick, so it just means "big dick" but with some fun wordplay added in. As for what power it wields, idk about the specifics but dual cultivation means using sex to exchange and magnify vital energies to become stronger, and is based on (at least theoretical) ancient daoist practices