Don't be so pharasaical about it. Yes, talk to your spiritual father about it, but for God's sake (literally), honor your grandmother and don't alienate your fiancé... especially if she
decides to go to your
services. I suspect that this is what your spiritual father will tell you. The rules still exist, but our church leaders choose how and when to enforce them. In this day and age, it's pretty hard to avoid inter-denominational families, and I'm pretty sure most would permit attending services for the purposes of honoring family, maintaining familial bonds, etc.
There is a line that you shouldn't cross: don't accept communion from another church. Every authority agrees on that.
An anecdote: when I was an Orthodox Jew, a Roman Catholic man would sometimes attend our (very traditional) Saturday morning services. I happened to answer the door when he arrived for the first time. He wore a kippah and a talit, so I assumed he was another Jew looking to worship. I spoke with him as I guided him to the synagogue; when he revealed to me he was Catholic, I did a bit of a mental double take. I asked him, "Oh, what brings you to the services?" He said "Well, my Lord was a Jew and He worshiped this way so I figure I should see what it's all about." The man probably didn't talk to his priest about it, and his garb suggested he might have been dabbling with Judaizing, but he was always very respectful and never
proselyted. In fact, he always left right after the services. I hope he's doing well!