Found: YA fantasy-ish book with mythology themes/characters

I think the main characters were two siblings. They showed up at their cousins’ house which was mossy and in the middle of nowhere. The time period is unspecific but contemporary (definitely not historical).

They ended up wrapped up in this thing called The Game (I think their cousins were involved with it). It involved travelling between planets and/or dimensions, but the book was definitely more fantasy (not high fantasy though) than sci-fi. The whole tone of the book was vaguely mysterious.

They discovered that their father was Sisyphus (I’m certain of that part), and their mother might have been Medusa. And the parents might have been stuck somewhere on different planets/dimensions?

I think the cover was mottled dark grayish, maybe with a whitish cloudy sphere in the middle. I read this sometime in the early 2010s.

Some of these details might be my brain trying to fill in gaps, but I’m certain about a thing called The Game, the space/dimension travelling, and that they discovered that their father was Sisyphus.

>1Sounds like The Game by Diana Wynne Jones.

Yes!!! Thank you so much! I've been looking for this for ages. It looks like I got a few details wrong (probably conflated a couple books), but I think this is it.

>3 castanea12: Oh good - I'm glad you found it!