Sharon,

Sorry for the delay in answering.

This is not so much a matter of your attendance being sin or not.  It's a matter of providential prudence. 

Your grandson, through no apparent fault of his own, was not brought up in the faith, so he can't be held to the same standard as a well-formed Catholic.  

So, it's not necessarily a sin, if you go as a passive participant to show your love and support for your grandson, on what is an important day in his life but, if it involves some kind of other religious ceremony, you can't actively participate in any religious rite.  And even then, use your judgment.

If it’s some kind of Christian/Protestant ceremony and the congregation asked you to join in saying the Our Father or as they may call it, the Lord's Prayer, it's not the end of the world. 

This kind of passive participation is allowed and tolerated by the Church.

In this case, it sounds like you're the only possible Catholic influence in your grandson's life.   You don't want to lose that connection. At some point your grandson may have a crisis of faith. He may remember your faith and come to you for help. But if you sever or hurt the relationship by not attending, it might give him the wrong idea about what Catholicism is all about. We obey the Church's rules, out of love for Christ, not out of fear.  

The flip side is that you don't want to appear to approve of his decision to marry outside the Church, so stay clear of the subject.  This boy, now man, simply doesn't know better. By going, you leave the door open.

Just remember why you’re there. You certainly want to celebrate his finding a woman he wants to spend the rest of his life with together. 

Pray for him and his bride . . . and pray for the boy’s parents. It sounds like they need it (prayer) more than anyone else. 

I hope this helps,