You know your friend better than anyone else. It is normal to worry, especially after such a great loss. I'm so very sorry for the loss of your sister so recently. You must be absolutely tapped for energy and emotions.
I was in a similar position, though the illnesses were not the same. My Dante struggled to breathe at night, would hack, couldn't rest comfortably, and would get more tired on his walks. He had developed a heart murmur that very quickly turned into an enlarged heart with a partially collapsed trachea. Most days he still wanted his walk, even though he'd be too tired and I'd carry him on the way back. He still wanted to cuddle, he had trouble eating sometimes but not all the time, and those nights were horrible, he couldn't rest, his chest would rise and fall so fast I thought I'd lose him then and there. But then again the mornings, he was up, wanted food, wanted attention, wanted that walk. Up until the very last night. I even thought he was on the mend, yet. He was gone the next morning.
The entire time, my gut told me what was true, and that I had very little time left with him. Even though the vet said he should live quite a bit longer, even though I saw him spinning around every time he saw me. The times that were not good were very not good, so I knew.
Please consider bringing this up to your vet if you haven't, though I suspect you have. Go through a quality of life checklist if you haven't.
This site has been so helpful for me these last three months since my pup left me. Everyone here understands. Everyone here has been there and is going through this with you. Just remember only you know, because you have that bond with Fluffy, no one else does. Listen to your heart and your gut and your Fluffy. They won't steer you wrong.