I am so so sorry for your loss
Most of the guidelines for the surviving rabbit are written for rabbits who are fully bonded. It does sound like your two were friends, but since they were not bonded I wouldn’t worry as much about the deep depression that can lead to physical problems.
I do think you should spend more time with her, as she is used to always having a neighbor around. Even if just hanging out in the same room with her, giving pets if she wants them. How is she doing? The main things to watch for are appetite and poops. It’s normal for grieving bunnies to be a bit more quiet than usual, but they should still have a good appetite and poop output.
I also agree with your instinct that she probably knew something was wrong with her sister, probably before you did. So hopefully it wasn’t as much of a shock to her.
I’m not sure if you are planning to get a second rabbit, but if you do notice her getting depressed to the point where you are worrying about stasis (which again, I don’t think will be a problem in this case), it might help to get another bun. Often it helps grieving bunnies just to have other bunnies around. You might even consider fostering one if you don’t want to commit to adopting yet.
Again I am so sorry for your loss. Give your girl extra snuggles, you can grieve together. <3
(((Binky free little one)))
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.