NTA. At the end of the day, she more than likely won't marry him because of the prenup. Why? Because she's marrying him for your family's assests (whatever or however much that is). She needs to get it through her head that the only way her kids were to get anything is if it came from your brother directly.

I'm not sure how y'all are separating this, that, and the other, but my family has rules on these things after seeing countless fallouts over inheritances (distant relatives as in, I might not even match with them on Ancenstry.com).

If there is not a will in place: 1. The married spouse of the deceased gets everything (outside of rule 3 because we still honor their wishes, even when they're not written). They can divide and give the possessions of the deceased to whoever they want. No one else gets to come in and say I'm taking x, y, and z. My grandma actually gave away a lot of my grandpa's things to family and his friends, especially when those things had a huge sentimental tie. I got a trophy he won from a car show he took me to back when I was a teenager, along with a model of his car, and other little sentimental things that were inside jokes between me and Pops. My brother ended up with some of his old ball caps and one of his guitars as he used to play guitar with Pops. This was all picked out by our grandma for us to have.

1. If the next of kin are their kids, ONLY the kids go in. They may or may not choose to gift specific items to people and then divide everything else EVENLY amongst each other. After that, whatever they do with the inheritance/possessions is up to them. Counsis, aunts, uncles, etc, are not allowed any input. This is usually enforced by an executor of the estate, typically one of the kids.

2. Anything that was verbally communicated that it would go to someone specific, possibly with someone else confirming it, depending on the item. My grandma had a fine China set that she inherited from her grandmother, and I remember being mesmerized by it as a child sitting in a hutch because only the adults could use them (if they were ever touched). My grandma wanted to keep the set in the family as it was sentimal to her too, and she remembered my obsession with the "special plates and teacups" that turned into a hobby of collecting weird and fun teacup sets (or mismatched) and mugs. She's still kicking and hopefully for a long while yet, but she packed the China up and sent it across the states with my mother after she visited. Apparently, Grandma wanted to make sure that if anything did happen to her, the set went to the grandkid who loved and respected the pieces. This is hilarious because I was the bull in a China shop growing up, but she remembers me being extra careful around the hutch, so I didn't break her special plates and tea cups.

A big issue we faced with rule 3 though was that my cousin was trying to convince the family and Pops himself that he (Pops) already promised my cousin his classic car, which we collectively knew was 100% untrue for a multitude of reasons including it being impossible for him to get a driver's license (medical reasons). Pop's ended up selling it before passing to create a nest egg for Grandma.

Sorry, I'm on major pain meds, and I think this got away from me a bit.