Expander
I can't say Ive been there done that but I do run OS 10.2.8 and can suggest a few trys. Have you tried opening the file in "Classic" yet using expander 7.0? have you tried stripping the extension .sitx to .sit and opening it that way (works sometimes) are you double clicking the file or drag and dropping it on the app. When I get stumped I look at everything sideways at least twice. and can you get the file in an .hqx format?
Good Luck😉