Must agree with T.Neo on this one. But i'm no expert in this and each situation is different for every family.
You said "I figured out that some things in life have to be delt with before life can move on" but then you said that this IS NOT the first time that such a thing has happened and that you will "let this blow over..."
You say that he's not a child abuser but your 6 y.o sister has a visible bruise on her back and you are scared. I don't know about you man, but in my book, that's abuse!
Perhaps calling the police on him is a bit extreme and it might not get you the results you want.
I'd advise you to confront him in a family meeting with your mother and your sister present and IN A CALM MANNER, discuss your problems with him, especially your feelings about him abusing you and your family and warn him (IN A CALM MANNER) that if he ever tries something like this again you WILL call the authorities!
I cannot STRESS ENOUGH that this conversation has to be made in a CALM MANNER! Do not show any aggravation or hostility. Make him know that you are the victims here and try to ask him nicely to stop doing that to you and your family.
If you see this conversation going the wrong way, if he starts to get mad and/or threatening, remove your family and yourself from the room and lock yourselves in another. THEN call the police.
I'm in NO WAY an expert in these situations man, i'm just telling you what i'd do if i was in a similar situation. There is NO way for anyone to put themselves in your shoes as each situation is different.
DO NOT DO any of the things you read here before you contact these numbers: I looked them up for you and i STRONGLY ADVISE YOU to contact them ASAP!!!
-National Society for the Prevention of Cruelty to Children (NSPCC) in England, Wales and Northern Ireland Freephone 0800 800 500
-Irish Society for the Prevention of Cruelty to Children (ISPCC) 00 353 742 9744
I found these numbers in this page:
http://www.kidscape.org.uk/professionals/childabuse.shtml
The people in these lines are PROFESSIONALS and they deal with situations similar to yours EVERYDAY!
They are the MOST qualified to help you in a situation like this.
I really hope that things will get better soon for you and your family!
Respectfully
Dimitris