@De Borah,
Your sister in her eyes will always be right, always have little emotions to share and always try to put people down, make them feel vulnerable.
Be assertive, make it clear how you feel, she'll argue the point but tell her you don't agree if you don't and walk away, don't allow her to control the situations.
On the same accord, communicate with her about "happy things" you never know, she may not show all of her colours and you may enjoy the time together if on a more light happier note.
Also I located this thread which may help as well..
http://able2know.org/topic/140331-1