This aunt’s Christmas Eve tradition is long-standing, and because your sons choose to attend it, I think you should accept that – for them – this is an important aspect of their Christmas celebration. And so, you should let them have it, and instead of hosting a competing event – you and your wife should scale back your own Christmas Eve and consider the way you celebrate it (with her sons) to be … what you do. Develop your own intimate traditions with the smaller group.
