SUBREDDIT: r/relationships
TITLE: Me [19 F] with my sister [20 F]. She's in a morally interesting relationship, and I don't know how to act around it.
POST: My sister was in a relationship with one guy for 2-3 years, ending long distance, and misery. She's been far more promiscuous and a little like a chameleon for the last year. She doesn't know how to be herself without someone there to appreciate it closely. She's been with a guy for a month now and they get along famously except he's in a long distance relationship with another girl. My sister says it's understandable because they have to know they'll work; he want's to be sure of my sister before he breaks up with his girlfriend.

I love my sister to the moon and back, but I'm afraid she's going to end up with someone who walks all over her. She's all wrapped up a guy who doesn't think enough of her to break up with a girl he dated for a week before college started. Whenever she talks happily about this guy she knows I think poorly of it. She thinks it's bad too but still won't change it. Should I try to talk to her about her life, or should I delete this post and back my nose out of her business?
TL;DR: 
I'm worried my sister is setting herself up to be walked all over and it's straining our sisterly bonds. I don't know if I'm too nosy.