Befriending Sirius Black would be extremely detrimental to her whatever was left of her sanity. And Marlene McKinnon, being her stubborn, sarcastic self, would not let that happen.
Ronald Weasley has always been the perfect idiot when it came to love. So when he screws up his realationship with Hermione, what lengths will Ginny go to to help him realize that he can't fight fate.