Wis. Stat. § 786.56

The validity of a sale made by a guardian may not be questioned by any person claiming under any title that is not derived from or through the deceased persons or the ward, on account of any irregularity in the proceedings if it appears that the guardian was licensed to make the sale by a court having jurisdiction and that the guardian did accordingly execute and acknowledge, in legal form, a deed for the conveyance of the premises.