Worried about my dad...?

my father has been a serious alcoholic all my life. ever since i can remember he would revolve his life around trying to get beer, liqour, ect and figuring out a way to sneak around and drink as much as he possibly can. he used to drive around in the car while chugging a beer with my twin sister and i in the back seat when we were kids all the time. about 2 years ago he got arrested for his 4th aggravated DWI (he blew a .32) and was forced to go to a 9 month rehab center. he did great in rehab, he seemed very determined/focused and would constantly tell me how sorry he was for being a bad father. after he got out he seemed like a whole new man! working all the time, taking care of the house, attending all of his probation meetings, he got his liscense back, ect. he was sober for about 1 1/2 years and then slowly started to go back to his old ways. i think alot of the reason for his drinking is his home life. my stepmom is very controlling, has major mood swings, shes very disrespectful to him, and ive seen her physically attack him many times when i was younger. their daughter (my half-sister) who is 14 treats him the same way. he does everything for them (cook clean, do laundry) while they sit around and watch tv and eat. he says he wants to leave all the time but never has enough to do it. i know hes been drinking again and i know its not going to end well. i dont want to just sit back and watch his life fall apart again but i dont know what to do? sorry its long i just wanted to explain the situation thoroughly