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. 

No, they are not equivalent. There are degrees to mortal sin just like there are degrees to venial sin. First‐degree murder is more grave than missing Mass, or viewing pornography, or any other grave sin that we might imagine. 
 
It is true that killing someone and missing Mass are in the same category of "mortal," (or grave) sin. But they are no more equivalent, than a rat is equivalent to or the same as a man, simply because they are in the same category "mammal." 
 
Your description of both sins as being punishable by eternity in Hell also implies an equivalence by that fact. However, a distinction is necessary regarding the way you connect the notion of eternity to punishment. That one is in Hell eternally, is not due to punishment per se. Rather, the eternity of Hell (or heaven for that matter) exists because, at some point, our decision for or against God, and the laws and values of his Kingdom becomes a decision that is for us at death forever fixed. Thus, that Hell is eternal, is not by itself a gauge of the punishment involved. 
 
We need not presume that everyone experiences Hell in exactly the same way, any more than we presume that everyone experiences heaven in exactly the same way. There may in fact be degrees of suffering in Hell, and degrees of glory in Heaven. 
 
While there are mysteries involved here, it makes sense that there are some Saints who, on account of extraordinary virtue, have a greater capacity to appreciate God's glory in heaven. It also makes sense that for those in Hell who have rejected God, and his Kingdom, there would be degrees of suffering experienced, related to how deep their rejection of the light is. Scripture indicates we are judged according to what we have done (Revelation 20:11‐15). Thus, there is at least implied some relationship of reward or punishment rooted in what we have done or not done. Jesus also speaks of places of special honor in heaven, indicating levels of some sort in the afterlife (cf Matt 20:23).