Maybe he has been 'over-punished'?
I bet if I took every one of DD/DS's toys/priviledges away they'd just get more stroppy with me, they have nothing left to lose.
One instance of bad behaviour means one punishment.
I do agree with what everyone else ahs said though, right now you need to get through to him, if that means holiding off punishments for a while then do so, just ignore, if he hits his sister, go to her, comfort her, tell him it's wrong then ignore him.
The punishments are going to be what is in the forefront of his mind and you need time to talk to him atm, involve him in anthing that takes you away from them for a minute (in other words, anything that means you cant watch his every move to protect his sister by stepping in), get him to hold a sponn whilst you cook or dust whilst you hoover, anything that keeps him away from them when there is no-one to supervise, that way you won't feel guilty about her getting hurt because you're trying not to punish.
Anytime he does be naughty just tell him you're not happy and ignore him, dont answer back if he does, and discuss it with him once he is calm. Hopefully after a week or two he'll get used to talking with you and not being punished(which of course if he has been behaing badly then he is used to) and will open up to you more.
Don't ask him to open up though, just make statements, not questions, for example if he says horrible things whilst he is 'stropping'(it's good to be able to label that in your head, because as long as you think , ok, its a power struggle, its a strop, you'll find it easier to ignore, because you will have a set response) then when he is calm, sit with him, and say, 'You calmed down now? I know you dont like it when mum tells you off, but I cant allow you to hurt your sister, ok?' hopefully he'll say 'ok' then if you want to ask him why he hit her, do so, but dont ask why he did anything he did whilst he was stropping, just tell him you understand he gets angry but you'd rather he didnt say because you love him and it hurts to see/hear.
He needs to open up when he is ready, and as someone else said, he may not even know himself.
Try not to beat yourself up about it too much either, I assume you too are grieving and trying to deal with this as well is going to be hard, but you're doing great, just by caring you're doing great.
xxx