How to kick out non paying tenant in Wisconsin?

My sister has an extra home that she inherited. It's a fixer upper type but has a new furnace and septic system as well as I think a roof. She let our younger brothers in law move in last summer. The in-law and his girlfriend were living with our brother at the time so my brother was happy to have him out. Since then he's only paid $150 rent in about 8 months and what's more he's stuck her with over $400 in electric bills because the electricity wasn't taken out of her name. There was no rental agreement in writing. How does she go about evicting him? She'd like not to involve the law but he and his girl friend are avoiding my sister.