If you and your brother are adults, welcome to the world. Harsh as it may seem, your parents have no legal obligation to support you any longer.
If you feel your sister is being abused or neglected in some way, notify the authorities. Same as with your siblings. Notify the police or child services in the appropriate jurisdictions where these abuses are allegedly occurring. Be prepared to provide details as to what you know and how you know it.
If you are trying to build a case to sue your parents, speak to an attorney. Most such cases don't go anywhere. But if your motives are to protect underage siblings, then call the authorities.