I find it difficult to believe all the fear and rigidity demonstrated on this issue. Yes the relationship is sinful, and you cannot support it. I think this is an excellent opportunity to engage in a dialog with your niece on your beliefs, why you believe them, and why they are important. It is absolutely NOT the time to be judgmental and “holy than thou.”
Attending a public ceremony may be viewed as a sign of approval, but if you look at the canons on marriage (not that they apply here, but it is the closest I can come to) the purpose of the other people at the ceremony (other than the presider, bride, and groom) is to be able to testify the event took place. That is why the others who attend are called witnesses and there is a requirement to have at least 2. Strictly speaking it is not to show approval or lack of approval.
When you eventually approach your niece and her family about this, remember when Jesus was found was with the prostitutes and tax collectors.
Some scribes who were Pharisees saw that he was eating with sinners and tax collectors and said to his disciples, “Why does he eat with tax collectors and sinners?” Jesus heard this and said to them, “Those who are well do not need a physician, but the sick do. I did not come to call the righteous but sinners.”(Mk 2:16-17)
*The Pharisees and their scribes complained to his disciples, saying, “Why do you eat and drink with tax collectors and sinners?” Jesus said to them in reply, “Those who are healthy do not need a physician, but the sick do. I have not come to call the righteous to repentance but sinners.” * (Lk 5:30-32)
You can love your niece, and even love her partner. You can express your disappointment at their choice to live in a sinful state. That does not mean you have to cut off contact with them, or pretend they don’t exist. Practicing the virtue of charity will go a lot farther in demonstrating to people you really believe what Jesus taught than anything else you can do. I hope if you approach your niece in charity, and explain your situation she will understand your reluctance to attend her event and respect your convictions.