How to convince someone?

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So here's the thing. My sister has some 'nasty' friends - as I'd call them - in not general meaning of the word, just she's carying about them so much and she's trying really hard to help them and they treat her like they don't know her then, they turn her off and then she's sad - you can tell. I was trying to convince her that she should hold on a little and so she should stop trying so hard, 'cayse where is the point really when they evidently don't want her help. I mean, I can understand that sometimes when you have problem or when you feel bad you don't want to talk about it and want to be leaved alone, but really, why they're treating her in this horrible way. She listened to my of course, but then she returned back to where she was, and I'm not happy about it 'cause she's sad. I'm thinking if there is anything more I could do...