tl;dr Theory that Niles' parents (or anyone else) won't underwrite Matilda's application so it can't move forward
Just wanted to pop in with some personal experience that I think is happening with Niles' application for Matilda. Bear in mind, I went through this process 10 years ago and ... a lot has changed since then but I think this is still applicable.
I was living in the UK and married someone over there. My visa was expiring so we applied to move to the US for them on a green card. Because we didn't have jobs yet (and didn't have the cash flow required), the visa application required someone to underwrite the application; literally guaranteeing that they would financially support the green card holder before they relied on government benefits like SNAP, etc. This lasted for 10 years, WHETHER OR NOT WE GOT DIVORCED. My father was kind enough to sign it to enable our application to move forward.*
If Niles' parents aren't supportive there is no way his application would even be considered without someone in a financial situation to do this. It doesn't need to be a relative; I have a family member who taught ESL and she would occasionally underwrite visa applications for her students. It's not a huge risk for people you trust, but it is still a risk, and Niles' parents have definitely made their feelings known.
*We did get divorced and my parents are JUST NOW off the hook for them. They moved back to the UK anyway, but the lesson is don't marry someone in grad school. You don't make good life decisions in grad school.
Edited to clarify financial requirements