I’m sorry about the loss of your bunny. You didn’t mention if the surviving sister has been spayed or not, but probably that hadn’t been done yet due to your rabbits’ ages. The reason the two sisters were able to live together peacefully, was because they were young and their hormones hadn’t come into play yet. At 4 months of age, your bunny will soon be dealing with that. Before you can bring another rabbit into your home, with the hope of bonding it with your rabbit, you will need to get your rabbit spayed. Any new rabbit that comes in will need to be spayed or neutered. You have to wait a few, or several weeks after their surgeries before you can start the bonding process. It is usually easier to bond a female (spayed) with a male (neutered) but you can bond same sex rabbits. Bonding can be rather quick and easy or it can take weeks or months, so be prepared for all that it entails. In rare cases, rabbits may never get along and never bond, so they would have to be kept separated for their whole lives. Fortunately, that rarely happens, but it’s another thing to be prepared for.

In the meantime, spend as much time as possible with your bunny. A stuffed animal to cuddle with may make her feel better, if she is not a chewer.

Do you know what her sister died from, and is it possible that it could be contagious? It’s very important for a rabbit to eat and poop. Have you offered your bunny her favorite greens? It may be a good idea to take her to the vet to make sure there is nothing wrong with her. I know that vet visits can be scary to a rabbit, but she may be in need of some Critical Care if she isn’t eating well. Or the vet may decide to give her something to stimulate her appetite.

I hope that she will feel better soon and start eating normally.

So sorry for the loss of your bun. Been in the exact same situation myself, and I made sure to spend a lot of my time with my bunny at the start hoping she wouldn’t get depressed. Unfortunately, your rabbit sounds quite affected, and if she’s not eating or pooping it’s important to get her to a vet as it might be gi stasis. Give your rabbit lots of toys, since she’s on her own now she will probably get bored easily. If you are thinking of getting another bunny, you need to get you rabbit neutered first and also think of all the other factors, as tobyluv mentioned above. You’d need to research a lot about bonding bunnies as it’s a very long difficult process. That shouldn’t discourage you though, it’s great to give a rabbit a home and also give your rabbit a best friend.