Rachel, you wrote that your father beat up your mother and then later your sister. Amongst all the emotions that must have be going on I'm making the assumption that you would have felt threatened that this could/would happen to you also. Did he threaten to beat you up or was it implied? Can you recall if there was something that your sister did that provoked him so he beat her up and not you or on that one night was she in the wrong place at the wrong time? Did you do something that meant you didn't get beaten up - avoid confrontation, stay out of his way etc.
How did you realise you had been manipulated and that physical abuse is physical? Had you always had a lingering doubt about it and did you still keep in contact with your sister or ever discuss it afterwards?
Please don't answer any of these questions unless you want to.
MMXIV