No, expat isn't a troll. A poster disagreeing with someone does not make that poster a troll. People who are giving their actual opinions are not trolls.
People posting bollocks to inflame - they are trolls.
People getting nasty and aggressive - they are trolls.
People just plain making shit up - they are trolls.
fwiw, if your sister is happy then you have to trust that she is an adult and is capable of making her own choices in life. It's not horrible to be concerned and it's not horrible to talk with your sister and make sure that she feels she is making the right choice for her.
I moved in with someone the day I met him. Married him 3 months later. Jacked in my job to move halfway across the country 2 months after that. Had our first child a year later and our second 15 months after that.
That was 16 years ago. We're still together.
I know people who dated for years, moved in after 5 years together - split up.
There are no guarantees in life.
If she feels sure then unless you think she's an idiot who is not able to make choices in life, you need to let her live her life. Make her own choices, take chances and yes, make mistakes if things turn out to be mistakes.
If it works out - great. If it doesn't - you'll be there to support her. You can't make someone live their life the way that it makes sense to you.
Perhaps she is even making this choice because she wants a child and that's ok with her.