The entire art of therapy is about finding a way to convince a person that a new way of looking at a problem, or a new way of responding to a problem, is both highly desirable and highly attainable. Iow, if you want to effect change, you have to effect change from within their own model of the world -- at the level of their personal values and self-perceived capabilities. You have to use their own reason and desires against them so that they are inwardly compelled by the evidence they already possess. This can be an extremely slow and tedious process and requires a good deal of sensitivity on the part of the therapist. Most advice falls on deaf ears because they don't share the values of the person giving it and/or they don't have the same opinion of their own capabilities.
So much this.
Relate to this a lot. My pure O OCD is invisible and it's endlessly frustrating trying to explain to others how much of a devastating effect it has on my life, especially as I am outwardly very capable. People just tell me "just stop thinking about it", which is kinda something I tried 20 years ago. There is often a frustrating mismatch between what other people assume someone can do, and what that person can do, or feels capable of.
In all likelihood
your sibling would like to be able to live a relatively normal life, they just either feel they can't or genuinely cant. It's extremely unlikely to be someone who actually doesn't want to live a good life, so giving pep talks to try to convince them to want something they already want isn't going to help. They just likely feel they *cant* get what they want.
They probably refuse therapy because they are either scared of it, or don't think it will work. They refuse social services because its usually extremely stressful and often people get treated like ****, and the massive stigma.
From the perspective of your relative they probably just see no way out
, so engage in escape to remove the pain. (source, personal experience).