Waterfall discussion

Hellooo! I'm trying to help someone find a book they read 10 years ago. These were the only details they could remember.
I'm not sure what time period this is set in.
I thought maybe Waterfall, but I'm not sure because I've never read it. I don't recognize it, but hopefully someone does! :)

Girl goes to live with family in Italy

Goes to a bookshop where a bunch of beautiful people come in, she makes eye contact with the main guy

Secretly they are elves/fae??

MC goes walking in some fields where she gets lost and comes across two women.

They lead her to a cave with a waterfall.. Ends up going through a portal to another world where time moves slower

She sees the guy again who is actually a prince in this world. He's a twin and they have some kind of abilities or powers.

She finds out she has ancestral connections maybe??

Another King ends up hosting something for her where he offers 3 gifts. 3 meaning engagement, but she only accepts 2

When the women were on their periods they had to leave because the men couldn't control themselves, but MC is unaware of this