I need to know about squatters rights?

I recently put my home up for sale,and have a buyer ,but my problem is one of my family members has been staying there rent free for a year,the pg&e is in her name,but the water is still in my name,I have given her at least a 3 month notice to move,now she has about 20 days to move,is there any way she can stop the house from selling?