Question: If she really never contacts me again after my very serious comment, does that mean that she really DID love me?
We don't know her and you should probably stop trying to read her mind and focus on YOU instead... .
From the experience I had with my uBPDxso she might have problems dealing with the sadness and guilt about what you said to her. And she might blame you for abandoning her and being selfish, thinking and acting upon about your pain rather than hers.
But this is all totally unrelated to loving you or not. Love is not the factor in what she does or does not.
There are probably greater anxieties in her mind that would overwhelm her even if she loves/d you and even more so, the more she loves you and the more you get closer the more she is scared to lose you, because this is the core of being BPD.
You threatening to hurt yourself because of your r/s is the ultimate proof for going to abandon her. And this is likely to send her running as far as she can from you. And yes, typically because she cares about you, she needs you, and she is afraid to lose you.
Focus on YOU brother.
Focus on getting stronger and putting yourself together.