in all honesty this probably won't go well and result in anything good because you have no real power in this situation. not to be a debbie downer but unless your mother is at least 100% on board and willing to back up any needed behavior changes you probably won't have enough influence to convince him to get help. most likely, it will just cause a whole lot of tension.

instead, i think your best bet is to be supportive to your sisters. build them up and encourage them and let them know you are available to them. spend time with them outside of the house away from him and help them with whatever their desires are. you can't rescue your sisters but you can be a respite for them.