yes you are right... for informal situation u can just use "kak" for kakak or "bang" for abang... and u can also just use "adik" to call a younger brother or younger sister.. adik : is used to address someone (woman or man) who is younger than you....

"Kakak" = is used to call elder sister in your family member (eg: sister, cousin sister) and also to address a woman who is slightly older than you. "abang" is used to call elder brother in your family member (eg. brother, cousin brother) and also to address a man who is slightly older than you. We also often hear wife call the husband as 'abang'.