Amy in the Uk we have specialist bereavement counsellors for kids and teenagers and it sounds to me like your sisters are in urgent need of some professional help.
Your Mum was the centre pin of a large family and losing her has obviously sent the focus of your lives spinning out of control.
I can only imagine your dads behaviour has been brought about by his own grief,but he really should be the one holding all this together and i cant imagine how you can cope with him going so badly down the wrong path.Its a huge problem you are facing and one you cant handle without a great deal of help.Do you have any other family who could talk to your Dad and try to make him see how much his family need him to step up to the plate and take control again.To lose one parent is a dreadful thing,but you must feel as if you have lost two.