I feel irritated on your behalf OP. And it’s not even over the trans issue. It’s over your sister’s attention seeking behaviour.
This is your new baby. This is your time.
This sounds on a par with relatives to object to the name you’ve chosen, or clothing choice.
This is the one time in your life, and the life of your baby, where you have proprietary rights.
Someone muscling in, as though they know better than you, whatever the issue, is tedious and annoying.
Personally, I would deprive the entire thing of oxygen. I wouldn’t mention it, I wouldn’t comment on it, I wouldn’t object to it. Simply ignore it. Almost as though you are indulging her.
The best way to deal with her is to show that she has zero impact. And, you know what, she should have zero impact.
Mentally file it away in ‘stuff siblings do that’s annoying’.
Congratulations on your new baby.