While I agree there may certainly be dysfunction in that family we aren't aware of, and the SIL is simply intolerable... but being that Evelin hasn't been clued in to exactly what this dysfunction stems from, can we speculate that perhaps Justin has always been removed from the family and difficult to get close to? I just don't think we can gather either way at this point if his mother is some master manipulator or if she genuinely wants to see her sons get close because Justin has always made it hard to do so. On the surface she doesn't strike me as particularly poisonous. I do feel she should spend more time getting to the core of why Justin feels and has made the choice to exclude his family, but if it's simply because the SIL is a huge PITA, I don't think that is all that devastating that Justin can't just grin and bear her.
All that said, who cares if he wants to elope? I was just recently married, and we were going to elope to an island to do it, but then decided to do a small ceremony with just family. I am very close to my family and my husband to his, so we wanted them there, but when we were bouncing around the idea of just going on our own to do it, everyone was supportive and just wanted us to be happy and have the day we wanted. It would be different if they were planning a ceremony with an audience but then specifically exlcuded some family members, but that doesn't sound to be the case.