I would just leave it as the FB announcements and any photos you share. Your friend can leave comments or send you an email and you can go from there. I don't think you need to send her an email or call. You could IM her next time you see her online and say hi, how are the twins, etc. Just leave comments on her FB. Whatever communication evolves from that is good.
I'm mystified on this horrible thing you said. All this time you had no clue you did anything wrong. Given it's third hand, I would try not to put too much stock in it. For all you know, your friend's mother's sister was offended by something while your BFF thought nothing of it,or said, "I can't believe she said that," without knowing the context of what got said. You just don't know. I've been in this situation, you have no idea if they're mad at you. I would just say to keep things open. You're a little hurt she didn't specifically tell you she was pregnant, so you feel you should tell her. This is YOU, not her. She obviously doesn't feel the same, so you can sort of follow her lead on where this friendship is (and I realize you were included on the FB announcement that got eaten). This friendship can evolve or not.
I have a couple of old friends where we connect sporadically. We sort of pick up where we left off after months and years of silence. The most communication we have is FB and keeping up with each other's lives is FB. Just see where things go.