Being an Emotional Sponge

I've heard that's an ADHD thing. Is that true? My sister is the most whiniest person I know. And her complaining makes me miserable. I feel empathy for other people, too but listening to her problems is much worse. I just fill with pessimism and hopelesness like a dementor's just visited from Harry Potter. She's kinda person who needs to talk things to get over them and after she traumadumps she feels fine and I'm the one who's suffering. We don't talk much but I live with my parents and she talks to my mum everyday and I can't avoid hearing. I really hate this.