Thanks so much for sharing these feelings and thoughts, Matt.
I'm very sorry for your sister's boyfriend, and glad to hear he's a fighter with loving support.
Keep on letting yourself feel the lows. As shitty and uncomfortable as grief feels, it's important to let it in—to experience the far corners of human feels while they're temporarily here—as you're doing.
Sending you strength and comfort. Trust that you will return to normal in time, and remember grief is something that connects us all. ❤