My nephew and his fiancée, both Catholic, and despite being warned, are planning to be married outside the Church. Can I, and other family members attend the wedding?

No, you ought not attend. Both of them are bound to have their marriage witnessed by a priest or deacon in the sacred setting of the Church. In celebrating the marriage outside the church without permission, they are entering into an invalid marriage. To attend, and to celebrate with them, signals support of this sinful action. 
 
While these sorts of situations are awkward, you are not the source of the awkwardness, they are. A firm line is appropriate in such serious matters which underscores the sinfulness of the situation. 
 
Your explanation to them of your incapacity to attend should be done charitably, leaving the door open for further discussions leading to convalidation in the near future, should they still go forward with their plans. 
 
Finally, avoid harsh debates with other family members who may still go. While attendance at such weddings is strongly discouraged, Church Law does not absolutely forbid it given the human complexities involved in such situations. Some respect for prudential judgments that differ is appropriate.