I think you could consider that Claudia's mood at home and her mood at college may not necessarily be equal. Like others have mentioned, family dynamics vs freedom away from that dynamic can affect mood tremendously. Also the behavior you describe is typical of teenagers and young adults (not all, but many). Claudia is maturing, yes, but she's still very young. Going to college isn't going to automatically make her act like a mature adult. This doesn't justify her behavior, but it may help you understand it.
If I were Claudia, I would not appreciate my parents complaining about me to my siblings (and it would likely show in my attitude). I think it's one thing for a parent to go to a sibling and say "I'm concerned for Claudia and she isn't opening up to me. Has she spoken to you about anything? Is she ok?" and another entirely to go "I'm really frustrated with Claudia's attitude. I don't like her moods and she needs to snap out of it."
Like others have mentioned, your sphere of influence is really only as far as how Claudia treats you and I think it's ok to address that with her. Use words like "It hurts me when" or "I feel like this when you say/do that". I would not say what your parents have shared nor would I go into the conversation assuming that she's the same way at school and has no friends. You can ask her about school and the people that she hangs out with there...as a sister, I think it's a perfectly reasonable converation to have with her. From that, you can guage whether or not this is a systemic attitude problem with her in general, or just the way she is at home (and then maybe try to explore what the dynamic that is causing that is...if it's just a function of her age and this awkward young but grown up stage or something that should be addressed with the family).
POD to this. Without specific examples I'm having trouble coming up with wording suggestions, but certainly if she says or does something that hurts your feelings, there's no need to hide that from her, or to keep up the interaction. "It seems like you're not in a good mood right now, maybe we can continue this game later," or "It seems like you aren't really having much fun here, maybe we should just go home," or "What you just said offended me, I'm going to hang out by myself for a little while," or something like that. It's good to practice these things beforehand, because (at least for ME) it's really tough to say them without adding a snarky spin, which doesn't help the situation.
And then at some point when things seem calm, you could ask her about her overall feelings. "You know, lately you've seemed really down and negative. Is there something that's stressing you out?"
To me her behavior as described doesn't sound that strange, for someone in her general situation. 20 is a tough transitional time--not a teenager anymore, but still quite young; possibly still dependent on parents financially, but used to being independent in behavior when away at school; possibly realizing that the goals you had when you were a senior in high school signing up for college are not the same goals you have now.
I supervise undergrads at work who are right around her age, and a large percentage of them end up changing their majors halfway through school, or at least changing focus from one area to another. This can necessitate a lot of stressful changes, both for themselves and for their parents (like staying at school an extra year to catch up on now-required classes = more money from parents for tuition). That would be the first thing I'd suspect with Claudia (barring other info)--maybe she's suddenly realizing that she doesn't really like what she's spent the last three years studying, and is brooding about what to do about it.