Your situation is one that is increasingly common, for an unprecedented number of Catholics are entering marriages that are invalid for one reason or another. In some cases, it is due to a decision by the Catholic party to have a non-Catholic minister perform the ceremony. Others are more complicated, such as divorced persons who marry without first seeking an annulment of the previous marriage. Whatever the reason, Catholics who take the Church’s teaching on marriage seriously are then faced with the dilemma: Should I go? Am I even allowed to go?
There is no official “one size fits all” policy on the part of the Church. Consequently, you will get different answers depending on who you ask. Some priests would say that under no circumstances is a Catholic ever allowed to attend a wedding in which the marriage is invalid, since to do so would condone sin and give scandal. Others (including me) take a more nuanced approach. Every case is different and requires prayerful discernment before the Lord.
Let’s take your particular situation as an example. Clearly you agree with the Church’s teaching and don’t want to give scandal. It is important to consider the potential consequences of whatever you decide. If you don’t go, would it so deeply hurt or embarrass your granddaughter that it results in a permanent rupture in the relationship? Would that be the most effective way to witness the love of Jesus to her? Another option would be to tell her that you love her unconditionally and will stand by her always, but that you are concerned that her marriage won’t be recognized by the Church. You could share why the sacrament of marriage is so important. (She may not have a good understanding of the Church’s teaching.) Finally, you could tell her that should she ever wish to get her marriage recognized by the Church (if that is possible), you will assist her. Under those circumstances, having shared your beliefs and offered to help her in any way you can, you could attend the wedding if the alternative would be to inflict great pain on her or even destroy the relationship altogether.
I suggest you take some time praying with the Gospel. How would Jesus of Nazareth approach your granddaughter’s wedding? Are there any instances where He attended celebrations hosted or attended by people not in compliance with religious or moral law? Ask for His guidance as you discern what to do. As far as the invalid marriage itself, it is possible you will not see this situation resolved in your lifetime. If that is the case, it is important to remember that Jesus’ love for your granddaughter is even stronger than your own. Trust in His love and try to be at peace.