As-Salamu Alaikum - I'm a convert and my sister's marriage is in trouble; how can I help in an Islamic way?

As-Salamu Alaikum. I'm a convert living far from places where Islam is common. I recently learned my sister is going through serious marital problems; there seems to be a lot of mutual disrespect between her and her husband, and their young son (under seven) hears their arguments. A few important facts: they are not Muslim and they married before I became Muslim, so there was no wali or mahr ceremony in the way Islam prescribes. I'm her brother, in my forties, and my father is too elderly to act as a wali. I'm trying to figure out what my role should be from an Islamic perspective. A couple of questions and concerns I have: - Since they aren't Muslim, does Islamic guidance about the wali apply retroactively or at all in this situation? - Even if I am not formally a wali, do I have a religious duty to step in as a mediator or protector for my sister, especially considering the child is affected? - How should I approach a delicate family situation when my cultural background isn't rooted in Islamic family norms, so I don't have the usual community framework to rely on? What I want is to act in a way pleasing to Allah and practical for my family. I can try gentle mediation, encourage respectful communication, and prioritize the child's well-being, but I'm unsure whether I should assert a more formal guardianship role or seek Islamic arbitration. I'm also concerned about overstepping and making things worse. Anyone with grounded Islamic guidance or practical steps for someone in my position - a convert in a tiny Muslim community trying to help a non‑Muslim sister with marital conflict - I'd appreciate your advice. JazakAllahu khairan.