Personal question please help

So this is about my dad and I can't find answers anywhere and thought you can help. So he's in his 60s and I'm 19 and my sister is 10 and my mom is in her 40s. Ya arranged marriage. But he always talks to himself idk what he says. He doesn't care about helping around and doing anything. Unless he's asked. So no responsibly. He doesn't understand stuff either. What can it be ? He's kind of slow. It's like he doesn't care about anything. And him and my mom aren't really close either. Please give answers. My mom has to do work as a mom and father.