mjangelvortex
Lady of Ships and Birbs
The Journal of Two Sisters did have Celestia and Luna getting their cutie marks when they were older so your headcanon for your OC has some weight to it.
She has no cutie mark yet but she’ll get it… eventually. She’ll be a bit of a late bloomer, even for an alicorn. I’m headcanoning that natural born alicorns take longer to get their marks, like teenage years or even as young adults.