YABU. The "reason" your friend gave is obviously only partly that you're pregnant, and mostly that, well . . . it's her sister.
You say you would have chosen BFF as your MOH over a sister, if you had one. But that's a ridiculous hypothetical. You have no idea how actually having a sister would have changed your relationship with your BFF, or what this imaginary sister would have been like. So there's no point feeling hard done-by with all the "oh, I would have done it differently" stuff.
You don't have a sister. You don't understand that relationship. There could be any number of reasons your friend made this decision. Maybe she was strong-armed by family. Maybe her sister is going through something. Maybe she sees this as an opportunity for them to bond? The whole situation is probably more complex than you can grasp. Especially as you seem to think how well they get on is an accurate gauge of how much they love each other, and you can't tell people you don't have any sisters yourself without adding the pity-me sadface emoji. That's weird, and suggests a tendency to make things all about you.
All this drama is stupid, frankly. "I'm furious", "our friendship will never be the same again", "I thought we were closer than this" 🙄. You either are close friends or you aren't. If everything else between you is as good as it ever was, then her MOH decision clearly isn't personal, and you're massively overreacting. Accept the reason she's given you and move on.
One final thing. Is there anyone else you would have wanted to be your MOH? Were you happy with the experience BFF gave you for your wedding? If so, then stop feeling so hard done-by. Yes, it would have been nice to return the favour. But you still shared a beautiful moment in your friendship, and have the opportunity to share more in future. Talking about it as a "bargain" and an "honour" the way you do makes me wonder if your feelings are actually hurt, or if you just wanted your moment to claim Irrefutable Best Friend status in front of everyone. It's fine to feel a little hurt, but you're letting your resentment get out of hand.