What and why exactly is it that you want to understand about people? Is there something troubling you? You can start by observing their behavior. Please consult a Psychiatrist for further information.

Are you newly married? It takes time to get adjusted and understand people around you when you are in new surroundings. I would suggest don't be quick to judge and don't assume things. Communicate clearly. You and your in laws (and husband) may be having different habits and ways of thinking and doing things and might not like a few things about each other. It is okay! You can communicate. You can observe their behavior and talk to them. I hope (and would like to believe) that they do not have bad intentions for you. You can consult me or any mental health professional if you need further help. All the best!