Welcome to BPDFamilies, happy to meet you, sorry you're here.
The silving lining in this cloud is that he's been properly diagnosed, so the only remaining hurdle will be seeing whether he's willing to try therapy or medication-for-the-symptoms (many BPDs will prefer to self-medicate with drugs or alcohol).
BPDs can have difficulty distinguishing the different types of "love" that normal people feel for one another, just like they have trouble distinguishing between "sad", "scared", "angry", "frustrated" or "anxious" sometimes. It sounds like he legitimately loves you and cares about you, but then felt that his role was threatened by your boyfriend. You grew into a normal healthy person who has outside relationships, he seemingly did not.
What can be most frustrating internally is the sense of hating your sibling as you mention. I grew up with a very strong "We're all one family, we all love each other and do anything for each other" mentality except it was undermined by an older sibling who was mentally unwell and preyed on their younger siblings sadistically to satisfy their inner deficiencies. We each learned to cut that toxic sibling out of our lives, some of us by adolescence and others not until adulthood - and like you we each received flak from our parents for it at the time, "Oh it's not fair", "Oh they're only like that because you aren't nicer to them", "Oh they'll stop sabotaging your relationships if you if you will invite them out with your friends more often".
To make it more ironic, that sibling (now about 40 years old) recently filed in court alleging that they had always been our protector and that they believed our lives must have been very overwhelming and scary when they were no longer in the house (we each, as adults, filed affidavits saying our sibling is quite mentally unwell and that we considered our lives to have substantially improved when that sibling had been taken out of our family structure...it's honestly painful to write that sort of thing in a document you know your sibling is going to read, but they were seeking to damage more innocent people). Basically I'm saying you'll want to look into some self-care, whether that's meditation/prayer or therapy - because it's likely going to give you a mental ulcer walking around hating a sibling even if they deserve it fully.
Getting back around to BPD and where your brother is coming from, the simplest explanation is that BPDs entirely lack a stable sense of self (often also lacking a stable sense of past/future existing only in the present) and are smart enough to recognise they are deficient in that way...which makes them incredibly insecure since they worry everyone hates them and is planning to leave them (ironic, right?), and they're not able to moderate their feelings and often not their actions either, meaning they often become physically violent towards themselves or others as they exist at either "0" or "100" all the time. (See, that run-on sentence ended eventually)
