Hi Idul *hugs*
Congratulations on sticking with NC and I'm glad the threats etc have subsided. I would echo what has been said though.. contact a DV organisation. They have all the tools and support to help you stay as safe as possible.
Your sister, your step-father, your friend. None of these people can you fix.
Your sister requires expert care: all you can do is be as supportive as you are able.
Your step-father requires expert care: all you can do is be as supportive as you are able.
Your friend requires expert care: all you can do is be as supportive as you are able.
Right now, you have your own stuff to take care of and your young children. It is not selfish to put that first. If you have anything left over in the tank to give, then that's up to you to give it, but your emotional reserves are a little like a bank account. Keep withdrawing and you will go into the red. Keep going in the red and you will collapse.
Do the people you mentioned know what you are going through right now? If so, then maybe a gentle reminder that you have a suitcase full of issues of your own right now, but that you will be as supportive as you are able, might help (?). If they don't know, then the same applies (giving as much detail, or not, about your sitution as you wish).
Tx