Maybe the FBI would be a better contact in this case? I may be wrong, but to me the brothers explicit words of intention and access to guns seems very much like a threat of premeditated domestic terrorism. If that's indeed the case, here's the FBI's page with contact options near the bottom.
I'm so sorry you're having to deal/live with this OP. You deserve much better. This internet stranger is proud of you for doing all that's within your power in such a shitty situation!
I think it's in reference to how bi people are often told they are being "greedy" for wanting all/both sexes rather than just having the '"one" the speaker expects. This usually then leads to the misguided conclusion that they are promiscuous and unfaithful - https://lgbtqiacounseling.com/bisexual/bisexual-stereotypes/#Bisexuals_are_Greedy_or_Unable_to_Commit