I have to agree with this.
You should have tackled her statements wrt marrying her brother and living next door as soon as you noticed this sort of thing was persisting.
You need to take a deep breath, and sit your DD down. Maybe take her to a park, sit on a bench together, eat a packed picnic lunch. You need to tell her that she is too young to be considering herself anyone's girlfriend and that the boys she has been bothering are not her boyfriend. You need to tell her the talk of marrying her brother has to stop too. This must be very weird and unpleasant for her brother. It's not cute or sweet. You need to point out that her behaviour has an effect on others.
Explain that she can have friends who are boys, and she can love her brother, but that expressing love in terms of marrying is inappropriate, and not what love of her brother or friendship with schoolmates means.
You need to stop any exposure to media featuring this kind of material. No TV shows, no films (including Disney princess type with a handsome prince or characters ending up married/ kissing), no pop music or music videos. If her father is in the habit of calling her "Daddy's little princess" or similar terms, maybe examine the impact this may have. (Ignore if not applicable).
If she hasn't already been told the facts of life, I recommend The Care and Keeping of You, published by American Girl Publishing. There are several books as part of this series, dealing with puberty, friendship, growing older and hopefully wiser and kinder. She needs to understand what lies ahead, physically, emotionally, and socially.
Ask her teacher to keep a close eye, and to report to you if this behaviour continues.
I'd be very inclined to do whatever it takes to get her into sports or some other demanding extra curricular activity so that she will start to value relationships that are not based on romantic affection, and value people for traits not linked to romance.