How can I support my sister in a troubled marriage involving her husband's mental health?

or she knows that its doomed , she meet him 10 or more years ago online and had never meet him before that she would of been mid 20's he was 40's. he seemed normal and had a good job in home heating repairs although over time he left it and went on to work on his own. so they got married and had 2 kids and seemed to be living the married dream i guess although more recently its become obviously something is wrong with him mentally. whatever caused it i'm not sure but he makes weird phone calls to our parents and other relatives accusing them of stuff then calls back a few days later to apologise. supposedly he is trying to see a counsellor but not sure if they have seen one together yet. i'm not even sure if still works much or what he does some days as he now works on his own and would only make $ if he got jobs on his own.

How can I support my sister in a troubled marriage involving her husband's mental health?
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