A. This depends on your mom and your age. If you're 12 vs. 18 - your emtional maturity is a big factor. In practicallity, the thing you can do is talk to him.
B. This is bad for you, your sister, and your mom. Emotional abuse is still abuse. It will still screw you up, emotionally.
C. Only your mother can answer this. Your opinion should NOT be involved as to whether or not to divorce. Your opinion about how you would feel if they did divorce is perfectly legit for her to ask. Don't get dragged into your parent's marriage - they need to work things out for themselves. You don't want to hear your mom say "I did this for you" ... she should stay married or not for herself. Divorce won't change the fact that your father is your father --- it has a huge effect on the fact he would stop being her husband.
D. Yes. The bigger question is whether or not he thinks he needs help, and whether or not he would seek help, and whether or not hewould actually accept it if offered.
Depending on how old you are, I would recommend you sit down with your Father and tell him how you feel. If you aren't completely comfortable doing so alone, ask your mom (not your sister) to sit with you. Be sure you stick to JUST your feelings - how does it make you feel being yelled at? How does it make you feel when he yells at your sister? Are you afraid this will escalate into being hit?
Sticking to just your feelings prevents your father from logically arguing or being defensive. He might try anyway ... but it is harder to argue with "I feel afraid" than "you're mean". Avoid anything that might be perceived as judgmental.
Try writing your thoughts and feelings out on note cards - this isn't politics - it's better to have crib notes than try to wing it. This will help you stay in focus and avoid being diverted onto other topics. This will also help you determine how, exactly, you feel about it.
Stick to 3 points, maximum, for this conversation. Don't overload him.
Don't try to solve everything in one sitting. It ain't gonna happen.
Tell you father up front that the only purpose of talking to him right now is to let him know how you feel. Be clear that no response is needed from him on the spot. Then stick to this ... don't start dragging other stuff in, and don't demand he respond. Don't start off issuing ultimatums.
Wait a couple weeks, and then bring the issue up again. This gives him a chance to digest everything; it keeps him from feeling like he's being interrogated; it lets him consider your words and have a clear, well-thought out response to you.
It is more important where you go, than how fast you get there. Taking your time, so that everyone involved gets a chance to air their feelings first will go a long, long way to helping reach a mutually satisfactory solution.
Ultimately, everyone should seek counseling. If your father won't go, you can't make him. His choice shouldn't prevent you, your sister, or your mother from going. Accept that this is going to be a long road, whereever it takes you.