Number one factor is the local body of elders. If they like you, they'll consider all mitigating circumstances in your behalf. If they dislike you, they'll disregard circumstances and throw the book at you. In some cases they'll consider circumstantial evidence, such as you were alone with her and could have done it, and in other cases when someone denies it, they'll say were there aren't multiple witnesses so there is nothing they can do.
Subtle details become important when multiple people are involved in bad behavior. Who was the instigator? Did the man take advantage of her? Did she throw yourself at the man? Even if it's consensual, the elders will want to assign most of the blame to one person. They'll consider the other person weak. (And honestly in that environment, it usually turns out that way. One person took advantage of the situation even if it was consensual.)
I have several personal experiences but they were not directly related to your situation. In one of them, two guys (not me) were caught smoking marijuana together at a quick build. For the JC, it became important who brought it, and who tempted the other to smoke it. They both said the other guy did it. Nobody will ever know except those two individuals. But the elders took what was presented them, past history of both guys, and what families they came from and chose sides. One was publicly reproved, the other just lost his privileges.
It turns out less than a year later, the guy from the respectable family was arrested at school for possessing and giving marijuana to his classmates. So the elders may have been wrong in their judgement. Once again probably, but who really knows. (I do because I was friends with them, and it doesn't really matter because neither of them were clean.)
In another instance,a sister in her early twenties brought accusations against a teen brother a couple of years after the fact. It wasn't sex, but if it wasn't consensual it was molestation. Still, she chose to bring it up a couple of years later after they had a falling out. He denied it, (I have no doubt he did it, but I'm certain it was consensual) but still got reproved for it and she stayed as a regular pioneer.
It all comes down to your local body of elders. It's best to deny it all together. But the two witnesses rule is applied unfairly. Plenty of elder bodies are all too happy to use circumstantial evidence or the testimony of one person against you if it suits their purposes.
My advice is, (notwithstanding your brother could be a complete asshole and deserves a reprimand, not saying he is, just saying I obviously don't know all the details) if he wants out of the relationship, then get out. If she brings charges against him, deny it. He's wealthy and well liked. The argument can be made that she's being vindictive and that by bringing these accusations, she's hoping that somehow it will make him get back with her or is just trying to ruin his reputation. If he vehemently denies it, it is doubtful that he will get DF'd. But he still could lose any privileges for awhile anyway. And there is always gossip.
