My girfriend wants to spend one night at least with me against her mother's wishes, can anything legally be done to us if she does stay over? 

There is nothing that your GF's mom can do here.  In most states, the age of consent to sex is 16 years old.  in fact, at the age of 17, your GF can legally more out of her mother house and live on her own.  However, you are in a politically difficult situation.  you do not want to upset your GF's mom as she is going to make all sorts of problems for you and her daughter if she financially support her daughter.  for instance, she may take your GF's car away or not give her money for school or food or whatever else.  you need to respect her mom's wishes as opposed to turning your head.  I suggest having a meeting together so everyone is on the same page - yes its hard, but you will look very mature.