I've been in a similar situation with both of my parents. My Dad was misdiagnosed with MS when he actually had a brain tumor. We had steeled ourselves for dealing with MS, but at least it wasn't something worse. Until it was. Mom died of Alzheimer's.
My best advice is going to sound overly practical, but here it is. Make sure all of your mother's legal/financial ducks are in a row - beneficiaries on everything. I can't stress strongly enough how important it is to have a durable power of attorney, advanced medical directive and medical advocate paperwork, all in order. Your Mom's decision on what she wants done with her remains are the most important and should be honored. If necessary, see if your Mom will convey, to your sister, what she wants. Getting arrangements all set and paid for, in advance, is a blessing when the time comes.
Check around for some support groups in your area. And, of course, you know that all of us on DU will be here for you, when ever you need us. If there are social workers available, talk to them. And, when the time comes, I cannot recommend Hospice highly enough. We used Hospice for Dad, Mom, my sister and my Aunt.
And spend time just talking to your Mom. Ask her questions about her youth. Keep her engaged. Both of my parents were 'disconnected' for the last months of their lives - Dad in a coma and Mom with her Alzheimer's. That didn't stop us from talking to and with them. With Mom, we spent a lot of time looking at old photographs. Even when she was lost in her own mind, there was some level of connection.
And play music. What ever kind she likes. The soothing powers of music should never be underestimated.
So take a deep breath. Make a list of things you think need to be taken care of and keep it handy so you can check things off. And never be afraid to reach out for advice.
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