SUBREDDIT: r/AskReddit
TITLE: I fear my sister is making bad life choices, but I don't want to be the overbearing big sister trying to tell her what to do with her life. Any advice?
POST: My sister is 18 and last weekend I helped her move into her freshman dorm. Her grades were very low in high school and she had to struggle to get into college, so my mother and I are very proud of her. Her college is a few hours away from our hometown.

A few days after classes started she called my mother to say she was moving back home. She said she missed her friends and was happy with her life and she wasn't like me (I moved 3,000 miles away from home for college and settled there).

Her plan is to transfer to the community college in our hometown (we live in a state with very few options for colleges) and move in with her (18 year old) boyfriend. She and her boyfriend were dating for one month before she left for college, though they've been friends for a few years. They plan to marry within the year and my sister will adopt her boyfriend's four month old daughter he had with his ex.

My sister is an adult and I don't want to tell her what will make her happy or how to live her life, but I'm concerned. My mother has told my sister she doesn't agree with her choices but she will not try to stop her.
TL;DR: 
My 18 year old sister wants to leave her college for community college and marry her 18 year old boyfriend of one month and adopt the guy's 4 month old baby. Should I talk to her about this?