Answer:
They each have a reading that repeats a sound.
七 SHICHI, nana, nana(tsu) - seven
弟 DAI, TEI, otouto - younger brother
係 KEI, kaka(ru) - relate to, concern, charge, duty
抱 HOU, da(ku), ida(ku), kaka(eru) - hug, hold, embrace,employ
掲 KEI, kaka(geru) - put up, hoist, publish, print