It is unfortunate that your daughter has chosen not to get married in the Church. I wonder what her reasoning is. Attending your daughter’s wedding outside the church out of respect and love for her is not a sin. We don’t have control over our children’s decisions, but we can be there to help guide them and do our best to shepherd them back to the faith. By attending, you are giving her an example of love and are avoiding giving her another reason to turn away from the Catholic Church. Not attending could do more harm than good and push her further away from the faith.
I am advising this way because she is your daughter. I do not know all the dynamics of your relationship with her, but I think this is the best avenue to take. If she were a friend or a niece or other relation, I would give different advice. Show her a father’s love. Be an instrument of God the Father and show her the face of the Father’s love.