The only reference I can find to the "format" is this thread from years ago...
https://www.mobileread.com/forums/sh...d.php?t=266365
Sounds to me like possibly some form of ePub with custom DRM that's readable only with some companies apps (kind of how Kobo will name files .epub.kepub). If that's the case then the OP is probably out of luck.
Hana, you can try renaming the file extension from .epub.sifri to .epub and opening it in Calibre's reader. If it opens and is readable then it should convert okay, if it doesn't open and throws an error or seems to open but isn't readable then it most likely has some kind of custom DRM which you'll unlikely be able to remove.