If she's growling and striking at anyone, that may be your answer to why she was dumped, and why she has obviously been neglected. The poopy butt may be because her cage hadn't been cleaned, and the cage hadn't been cleaned because the rabbit's possessiveness made her owner afraid of her. The same thing with the claws - how do you trim the claws on a rabbit that you can't handle, because you are afraid of it? My guess, is that the rabbit was a child's pet, and the child's parent finally got fed up with the animal's not being looked after properly. Maybe the parent was one of those people (and I have known
many like this) that think that releasing a pet rabbit "to the wild" is perfectly fine!
Cuddles looks like an oversized Netherland Dwarf to me. I breed Dwarfs, and IME, they often have outsized personalities! Some of the sweetest buns I have ever owned have been Dwarfs, but sadly, the few truly vicious rabbits I have known have all been Dwarfs, too. A friend of mine turned a handful of Dwarf youngsters out to free-range with her chickens. It was so cute to see those little rabbits hopping around, at least for the first few weeks. Things changed when the sole buck in the bunch hit puberty. One morning, as my friend was carrying some hay to her cows,
something came up off the ground and knocked the hay out of her hands. It was that little 2 pound buck, and he was in full attack mode. He latched onto her ankle (I saw the bite mark) and it was all she could do to shake him off! Things went downhill from there, let's just say that her husband dealt with the rabbit permanently.
I have seen a lot of rabbits (mostly does) that resent people coming into their cages. Growling and striking, charging and even biting is typical behavior. A dominant rabbit would "tell off" a subordinate that had come into its space in just this way. When Cuddles does this to your sister (or anyone else) she is saying, "I outrank you, get out of my space, leave me alone!" She is not afraid, she is being bossy (afraid is running, hiding in a corner, and whining). Like any other animal, rabbits will pretty quickly figure out who they can push around. I have had animals that were perfectly good with me, be quite snotty with my kids. Is your sister (a lot) smaller than you?
In rabbit body language, high is dominant, low is submissive. A rabbit that is (maybe) going to challenge you sits erect, and has its ears and tail up. You have to convince such a rabbit that you are more dominant than it is. When you put your hand into that rabbit's cage, you keep your hand high, and only pet it when it lowers its body and ears and submits. To put your hand in low (like you are trying to pick it up) is just asking to get smacked!
Cuddles is one lucky bun to have found you. With time, maybe your sister can become friends with her too. (Her color is Opal, in case you were wondering!!)