Why that?
OT: As far as I know, it is kind of looked down upon. But then it depends upon how cool the family is. In this situation, unless your MIL explicitly invites you over, I would go with what your mom is saying. OR if your fiance tells your mom that he wants to bring you over, and she is fine with it, then sure.
Obviously then, you want to hang out with the in-laws. If they leave the two of you alone, it is best not to leave the living room (i.e. to go to his room).