Honestly, I would say something right now, and be prepared to piss them off, because if you don't it's likely that they will spoil things in some way when you've just given birth which is a far, far worse prospect than anything else, including a stand-up row now.
When you've just given birth you'll be mentally and physically vulnerable. So any antics will not only have a potentially really bad effect on you, but will also be far, far harder to forgive. Plus, it's a magical time. Don't let anyone spoil it. If someone is shaping up to elbow their way in and spoil it, nip that in the bud right now.
Your sibling sounds pretty horrible. It's awful for them that they are desperate for a baby and can't right now have one, but that 'rip in half' comment is unforgivable. Really toxic. For that alone, I'd be saying - make sure you keep them WAY at arms' length around your birth and the weeks after.
But if it looks as if their focus has changed from hating to trying to muscle in and take over - ooooh no. Much worse. Yes, you need to nip it.
Where are the rest of your family in this? Supportive of you? Or just wanting 'you two to get on' for a quiet life, or scared of the nasty sibling? If they are at all supportive, have a chat with your closest relatives first. Be blunt. She has been bloody horrible, you are telling them right now that you plan to keep her at arms' length for a while around the birth because the alternative is likely to be an epic fallout. You'd like their support in helping managing her.
If not, don't bother. Family dynamics are difficult to shift.
But either way, I think you need to tackle her directly. And like I said, you actively NEED to be blunt to show her right now that you won't be pushed around. Especially as it seems that so far, to keep the peace, you kind of have - that comment would have meant an instant FUCK RIGHT OFF from me.
'It's not 'our' baby, especially not from the person who even a couple of weeks ago was saying they'd like me to suffer birth injuries. Can you stop saying that please? I know you've got mixed feelings about this pregnancy and I have tried to be understanding but I think I'd prefer the hostility to the takeover bid. It's my baby, and after the way you've treated me so far it's very much going to stay that way.'
And let her explode.
Then draw a line under it.
She will mind her mouth a lot more, and when she doesn't, you tell her to back off.
Then make damn sure with parents, whoever - that there will be NO unannounced visiting with her in tow unless it's cleared with you.