Marry your best friend and his family

Why is it that in laws affect your marriage so much if it's your mother in law or your sister in law. A relationship cannot just be between two people in love but many more that are involved. I feel that I take to heart what others do or don't do. Any advice to change that? Or is it something I just have to take because I married into it? Never really think of these things before head as your so in love with the man of your dreams.