First let me state that I am not licensed in MO and you should consult with a local attorney.
That said, if you don't show up and your sister-inn-law does, then you will be defaulted. If neither of you show up, then the case will be dismissed again. However, filing frivolous lawsuits is unlawful in most districts and your sister-in-law could be fined by the court. I would recommend that you show up and bring it up to the judges attention. I would also recommend that you speak with a lawyer about this if it happens again.