Fantasy/ pseudo-historical Romance. 2 different storylines: one with two priestesses in the ancient past, & one with their modern day descendant who is affected by a family curse after travelling to see either Mardi Gras or Venetian masks. Spoilers.

[Tiamat100]

Hi! I am looking for a book which I read around 2014-16. Might not be entirely accurate as my memory is a bit fuzzy.
The story features two different plotlines, one with two priestesses in the ancient world (my gut says in Egypt but I am not certain of this) who befriend each other. The other plotline is modern day characters trying to break a curse.

SPOILERS AHEAD

The two priestesses are meant to always wear masks to show that they are not individuals but representing the role they have. One of them is meant to die in a ritual and pass on her position, but the two attempt to escape. I think they fail and a line of descendants end up being cursed. Every few generations someone will be affected by the curse either in late teens or just after reaching adulthood.
This brings us to the second plotline, which is modern day and features two characters. I'm pretty sure one of them is named Charlie. After visiting either mardi gras or venice, one character fills ill, struck by the curse and dreams of the past. In any parade or celebration involving masks, a group of ghostly, masked dancing girls is seen, and Charlie's friend (the curse victim) is due to join them when she dies.
Charlie sets out to find out the history of the curse and figure out how to break it. I'm not really sure how it is broken, but I'm about 80% sure that either Charlie and her friend fall in love or that it was hinted at heavily. Eventually the curse is broken and her friend recovers. I think there is also mention of a tune that the curse victim hears, but I'm not certain of this.

I've been trying to figure out what this book is called. I know I read it in ebook form, but I can't find it anywhere. Any advice or ideas as to what it might be would be greatly appreciated!

If this post fails to follow the rules in any way please let me know and I will edit it.

[Tiamat100]

Remembered as well that the cursed character had a recent ancestor who had died at the same age as the cursed character. Possibly 21. There was a song that was sung by the dancing girls that had also been passed through the family. The cursed character may have been called Jo.

For the priestesses, one of them had no power and was not allowed to talk to anyone except in an official capacity as part of rituals. Neither priestess was ever allowed to take off their masks, but as they became friends they did. The curse was caused by a male priest who was furious at their attempt to run away and had wanted to sacrifice one or both of them.
The second priestess had slightly more power and freedom and encouraged the first one to break the rules.

Charlie, the modern day best friend of the cursed person, travelled all around the world searching for past victims of the curse to find why it had happened to their friend. I think that it might have been broken by a song, but I am not certain. I think the cursed characters grandmother was also helping. The cursed character, possibly named Jo, was hospitalised.
I think Charlie was basically an unofficially adopted part of the cursed characters family.

I think I read it by borrowing it on kindle prime library around 2014-15, but can't find it in past borrowing records.