Linda, My heart goes out to you. It sounds like your sister's condition is progressing rapidly. It is great that her son is coming soon. Has he seen her recently, or is he going to be shocked when he comes?
Is he the type that will step in and help with the legal matters? If so, can he, or you, or another family member locate an eldercare attorney near you and set up an appointment early in his visit?
It sounds to me, from your descriptions of events this month, that it may be time for Legal and Durable Medical Power of Attorneys, or even a guardianship. Encourage him to take an active role in this if he can - please do not add the burden of making critical decisions and dealing with finances to your already overflowing plate!
At the end of my Mom's life, even though she was still fairly competent, my brother held her Financial/Legal Power of Attorney and sister and I jointly held her Medical Power of Attorney, so we could make decisions together about her care (there are 6 kids, the rest were okay with the arrangement.)
Definitely invite him to all medical appointments so he can hear first hand what her doctors advise.
Please come back and chat here - we will hold your hand remotely as you travel through this difficult time.
Sue