Siblings who find a time machine portal

I started reading this book in middle school but had to return it to the library before I could finish it. For reference I went to middle school from 2015-2018 but I'm not sure if it was published around that time or earlier. The book was about 2 or 3 siblings(I remember their being at least 1 brother and 1 sister) who go to stay/live with a distant relative(uncle I believe) because their dad is in a coma. Their relative or uncle tells them not to go into the basement but the sister does anyways and finds a time machine portal that takes her back to different places in time. so the brothers has to save her. I also remember their being a weird room filled with mirrors or paintings. The only things I remember about the cover is the main color which was a light sky blue or robins egg blue and I believe there was a long word in the title like mysterious or something like that.
Does anyone know what book this is? it's driving me nuts lol.

>1 Fuzzyliongirl: The Girl with the Ghost Machine? It doesn't fit exactly, but it came to mind immediately

>2 RosetheReader: No that's not it