Fantasy: (technically a trilogy iirc) about royal twins facing an otherworldy invasion

[Roman]

Basic plot: A pair of twins (Prince and Princess)'s parents disappear/die (not sure), and they're appointed as Prince and Princess Regent of the kingdom. At the same time, they're facing an invasion from some type of elf-like otherworldly beings.
In the second book, the prince goes through a portal and ends up in their world. I never ended up finishing the second book or the trilogy.

Read somewhere between 2009 and 2012, library I got it from is gone, sadly.

[CBRetriever]

sounds like Tad Williams' Shadowmarch series (4 books)Shadowmarch.

For generations the misty Shadowline has marked the boundary between the lands of men and the lost northern lands that are the lair of their inhuman enemies, the ageless Qar. but now that boundary line is moving outward, threatening to engulf the northernmost land in which humans still live--the kingdom of Southmarch.

The twins Barrick and Briony, who in such evil times have only each other, may lose even that bond as darkness closes over them. As the Qar's power reaches out across their land, will Southmarch Castle, the only home they've ever known, become in fact what it has long been called--Shadowmarch?

if not, it's a pretty good series

[Roman]

@CBRetriever, yes, that's exactly the series!
Thanks!

[CBRetriever]

you're welcome. I totally enjoyed the series