Believe me Liz, you have to take as much time as you need. I personally still feel like I'm in a state of shock sometimes. Even though we are all on this site for the same reason, we all grieve in our own way and in our own time.
Liz, I am so sorry your life has taken this tragic turn. Each time someone new joins the group, my heart aches for them, as it's aching for you and the rest of your sister's family. Even though I am going through the same thing as you, I can't imagine what your sister's children and your brother-in-law are going through. I know you want to take care of them all, but at the same time you have to take care of yourself. Allow yourself to greive and express what you are feeling. Scream when you have to! Do not keep those emotions in check. It will not do you a bit of good. With this site we can read these posts and learn from each other's horrible experiences. You may read something one day that makes perfect sense and end up having a "good day". Come in when you are feeling despair and have no idea where to turn next..someone is always here to read what you have to say and give you the support you just may need. Please allow me to extend my deepest sympathy on the death of your sister.