I'm on my third marriage, the first two being absolute disasters. Looking back, I can see that I was not ready to be married either time despite being in one of the relationships for 7 years before I married him. I was also suffering from emotional dysregulation which made me hard to live with and knowing that, I put up with abuse because I didn't think anyone else would put up with me.
When I met my third husband, on the face of it, he was a strange choice too. We were given 5 minutes because everyone said that when I asked for Coq au vin, he would be asking for chicken in that red sauce. Actually, he is nothing like that, he is kind, well educated and will not be manipulated. My mother was horrified when he gave me the confidence not to be manipulated too! It is our 25th wedding anniversary this year and we have been together 28 years. We struggled the first couple of years as I found my way through the problems I had and I worried myself sick that I had made another mistake but I can honestly say he is my soulmate.
I would encourage your grandchildren to attend because if this is the marriage to end all marriages (in a good way) it will start it on a better note. Starting with conflict with other members of the family will spoil it. Encourage your daughter to listen to her children's concerns calmly and tell them why she thinks this is different this time. It may well clarify her thinking. If you get the opportunity, encourage your daughter to think about when she first had warning signs that her previous relationships were wrong. You don't have to make any noise about her current relationship but if you can get her thinking about the warning signs, she might think carefully about her future actions. However, you will have to tread carefully, remain calm and not get into an argument about her choices because you will just push her further into her current pathway.
At the end of the day, you just have to accept her decisions and decide for yourself whether you will be there to pick up the pieces. I found myself being bally ragged into telling my sister what I thought when she was making a very obvious bad choice to someone 20 years younger who just wanted a passport. She was insistent she wanted me to be honest and foolishly I believed her. Actually, she really wanted my approval and when I fell headlong into the trap, I think I started the ruin of our relationship which has never recovered. Being right was no consolation in the long run to any of the parties involved.