Engagements can change family dynamics.
My little sister and I got along great as kids. We played together a lot, and even when we made new friends and grew different interests, we promised to always have each other's backs.
In high school, I befriended and eventually started dating a girl my family adored, my sister included. After nine years, we have finally become engaged, but now my sister has grown hostile toward us. She never hinted that she disliked my fiancee before, and nobody in our family can get a reason from her.
There are multiple possibilities. One is that his sister liked things the way they were, as they'd been going on for almost a decade; the engagement and planning for a married life means things will change, and she doesn't want them to change. Marriage legally makes the spouses next-of-kin. Until he's married, his parents and sister are his legal next-of-kin.
A possibility that can't help but come to mind on this blog is that the siblings have a history of intimacy and affection that this newer relationship ended, with a wedding likely to close the door entirely. Maybe the letter writer's emotions from those times were not as deep as his sister's? Maybe he saw it as just youthful experimentation and play, while she saw it, and still does, as more?
Maybe the sister has wanted to have more intimacy and affection with her brother, meaning the siblings have had a
missed opportunity. That would definitely make sense out of her not telling anyone why she is "hostile."
It's possible the bride-to-be said something negative to the sister after the proposal, especially about how close sister and brother had been, and how that will never be the case again.
Whatever is the case, hopefully things will work out for the best.
Dear Abby advised that the brother talk with his sister one-on-one. That's a very good idea. But what if his sister confesses a strong
consanguinamorous attraction to him? He should be prepared for that, as well as hearing some unpleasant observations about his intended.
In general, anyone who is going to legally marry should have discussed with their future spouse what the rules of the relationship are and will be about flirting, sex, romance, and general socialization with others, especially if they want the rules to be different than they've been for the many years they've already been together.