we all grieve differently and there is no way to know what another person needs without asking. We also must be careful to not force our grieving on others.
My dad passed away suddenly 5 years ago. My 2 brothers (14 and 29) and I (27 and 4.5 months pregnant) grieved very differently. We were all very close with our dad and our worlds were completely shattered. however there were differences, like I was at peace because I got to say goodbye. My older brother (29) felt guilt for not coming home sooner. My little brother (14 at the time) felt at a complete loss because he never got to say goodbye. Partially because he was wrapped up in basketball and thought dad was just sick and would recover.
I guess what I am trying to say is that we each need something different. I know with my little brother, just checking in with him and making sure he was talking about things with friends was enough for him. Whereas my older brother and I found peace in reminising. Not something my little brother was ready for.
Try to find out what she needs and just be that for her. Just check with her and make sure healthwise she is ok. If talking about mom all day makes her grief worse, then don't talk about mom with her (for now)...talk about other things like what you love about having a sister. or take her to a place she likes to go. work on keeping your relationship with her as strong as it can be or even stronger. In the end thats what will be the most important. You can honor your mom by being the best sister you can be.
My brothers and I are so much closer that I ever thought possible. Dad helped us get to this wonderful place where we can cherrish eachother instead of just being siblings. We can talk deaply and tenderly with eachother and not just shoot the breeze.
I hope this gives you another perspective and can be of help to you.