Jag hittade historien - det var "We Three" av Dean R. Koontz, 1974, som visas i Konstiga motorvägar antologi. (Jag är OP men jag glömde min inloggningsinfo)
"We Three": three siblings with special powers eliminate the rest of mankind, thinking that they're the "new race", but soon one of them is pregnant with a creature even more powerful who just might eliminate them.