Is it right for my mother in law to "borrow" money and not pay it back?

My mother in law (who works full time) is such a slime ball. she is always "borrowing" money from my fiance and never pays it back. Meanwhile she has a job, theres no reason she needs to borrow the money except for her to get her nails done. It pisses me off bc my fiances does not say no and i feel like she is taking money from our household income for her own selffish needs. She doesnt ask any of her other kids for money bc she knows they would say no and she knows my fiance is a pushover. The week before she asked if she could live with us? I seriously dont understand why a healthy 50yr old woman who has a fiance of her own leans on us so much! Should i tell her to stop, im so sick of it. It creates so many arguements.