there are so many psychological issues going on here......the pathology of your sister...the pathology of you family......and, of course, your own pathology.
pathology is NOT a bad word....it refers to how we think and deal with situations.
it seems that your family keeps secrets.....from the world, from each other?
please, DON"T MISUNDERSTAND.....this is not a critisism....just an observation fron the limited info you gave so far.
and please, don't misunderstand this either, but the fact is, you may need to look more closely at yourself to find some answers, and to HF for SPIRITUAL guidance, which can transcend "worldly, psychological" guidance/ experience.