most advise cutting off contact because there wasn’t an immediate acknowledgment of responsibility for being hurtful, an apology, and a willingness to do better.

It's been four years. The problem isn't that there wasn't an immediate acknowledgement, the problem is that it has been four years and Sis has chosen to allow her son to continue to spew hateful vitriol. And LW is aware that this isn't a weird one-off, either.