Any details about the original file(s), the transfer media, and the modern equipment used?
It is usually difficult (or not possible) to decompress/open an old .sit (or .sea) file on a modern computer without damaging the contents.
In this case, the best way may be to look for a pre-1998 Macintosh computer with an appropriate StuffIt Expander version to be used for a decompression attempt.
Generally speaking, if old Macintosh files (compressed or not) are on floppy disks, make sure that each floppy is locked (write-protected) before trying to do anything. Make a copy (preferably, sector-copied) from the original (locked) floppy disk. Work on the copy only.