Should step-daughter be allowed to enter house at will?

Been dating my fiancee for 14 years, and step daughter has moved out to her own apt. When I come home she is sometimes in our house without our knowledge, but upon explaining her mom always seems to excuse the facts. I am never alerted to these frequent visits because she says it is her moms house not mines. I really love my fiancee and do not want to come off as asking her to choose, but I do how eget her to see that when our kids go off and live their life, then we are suppose to also.