Too often in troubled-families-in-denial, the sanest one gets dumped on. That old
dysfunctional family pattern (DFP) happens, with members getting pushed into various roles that perpetuate the pattern.
John Bradshaw psychobabble aside, these patterns really do get acted out…sounds like you’re the one who been assigned as the
punching bag/
eye of the hurricane. They can't vent on your sister, wouldn't be prudent; she might hit'm with the TV. And she's busy with her own role (for which her mental instability makes her well suited). By rationalizing your sister's violent, self-destructive acting-out as a
phase, your mother avoids a lot extremely painful healing work. Without the work…well, it all just goes round and round and round. Ouch.
Be this as it may, putting you in the role of ventee is totally unjust, unhealthy, and wrong. You need not stand for it, you certainly don't deserve it, and you have every right and responsibility to protect yourself. You probably already know this, but, being a survivor of a crazed family with a very psychotic central person myself, I know a troubled family can’t muster the support you need. So I root for your health, I root for your transcendence, your self-actualization, and I applaud your ability to access the situation and say, "This ain't right!"