Sister Ruth's mental condition
I'm fully aware making too much of the question is beside the point of the story. Her function is rather the effect she has on her surroundings, not focusing attention on the causes of her sufferings (about which the film has nothing to say).
Still, for the sake of interpretation, what psychiatric diagnosis is suggested by her speech, body language and behavior?
She's intense and emotionally unstable, on the verge of breakdown from the very start, making an effort to control herself.
There's a raging and overly suspicious quality to her jealousy, suspecting an affair between Dean and Sister Clodagh. Just dressing up "as if", and the blunt way she proposes, even the surprise when Dean turns her down, suggest that her thoughts of love between them lack reality check.
Borderline personality disorder? Paranoid personality disorder? Paranoid schizophrenia?
And indeed, she's homicidal, possibly suicidal. In other words, she's a loony. But what kind? Any ideas?