It could be that she’s having some feelings around having another sibling and is using the name as a more appropriate outlet to express this. Adding a sibling to a family is a traumatic event for a child already present; the child experiences dramatic and sudden changes in their life, a sense of loss of control, and disruption in their attachments to their caregivers. She’s done this twice already, and at seven, she may (consciously or unconsciously) understand what exactly another baby means–that she will get less attention from her parents, that her home will be more chaotic and noisy, that things will be very different once again–and is experiencing anxiety, grief, or other emotional discomfort around the news. Plus, the idea of new babies is generally presented as a joyous, wonderful thing to existing kids by parents, because that’s how the parents experience it; she could be feeling ashamed, guilty, or just plain bad/wrong that she isn’t experiencing it this way to boot.
By saying that she has to just “deal” with the name, you’re kind of telling her that she has to just “deal” with the baby; she doesn’t get a say in the name, just as she didn’t get a say in having another baby in the family. That could be making her feel pretty powerless, which is a crummy feeling no matter what your age. Give her some agency back! If she’s interested, make her a part of other things related to the baby: colors for the nursery, the outfit the baby comes home in, a say in the middle name, etc.
Finally: she doesn’t have to like the name! It doesn’t feel good to have your favorite anything tactlessly vetoed by someone important to you, even if that person is seven. You’re right: you made the baby, you pick the name. If you like the name, then use it! Her feelings around the name will pass.
I’d suggest sitting down with her and having a chat about how she’s feeling around the name and new baby. Validate her opinions and emotions. Tell her it’s okay that she doesn’t like the name, maybe suggest that she can call the baby a special nickname of her choice if she doesn’t feel like using the name you end up picking. Encourage her to be honest around the idea of the baby, and normalize feelings of weirdness–it’s okay if she’s feeling apprehensive, because change is really hard, and really scary! Be (gently) honest with her about how it makes you feel when she reacts so negatively to the name you love–it hurts your feelings! Ask her what she needs to feel loved and secure in this time of transition.
I work in children’s mental health and am in a grad program for counseling–just my perspective on this situation from the wee bit of information you shared. I apologize if I’m totally off the mark! Best of luck, and congratulations on the new addition!