Can my mother in law file for joint if she live in different state from her husband? She hasn't been working since last year and been staying with us.. Can her husband claim her or can I claim her? Can her husband file for joint even though he hasnt been supporting her?
You can only file jointly if you BOTH sign the return so, depending on the relationship, they both may have to file married, filing separately. On the Federal Return it doesn't matter much where each party lives, it's all about the married or single status.
I have no idea about the "who is supporting" her question - do you provide for her total support?