What a shame that your mother-in-law is so limited in her readiness to try to deal with this. What I know about these conditions is that there’s little that you can do when the other person is this way. The message to them is that you are ready to talk, but that you have no plans to turn yourself inside out for them. Explain that without her willingness to talk about it, there is nothing you or her son can do to make it better. In the meantime start planning your lives without her, but keep your offer of willingness to talk on the table.