I have APS. I'm adopted so don't have an answer about it being passed down. You are correct, it is autoimmune.

Actually, large numbers of people can test positive for anticardiolipins and NOT have APS.

It is only diagnosed after certain criteria are met: 2 or more positive labs for anticardiolipins (at least 6 weeks apart) AND either several miscarriages or a confirmed thrombotic event.

I was diagnosed after 3 strokes (within one week) and 3 labs positive for anticardiolipins over 18 months. I have also had two amaurosis fugax events, but both occurred after stopping my blood thinners for 48 hours (for a procedure or other reason).

I don't know if your sibling is being treated with anticoagulants, but the condition IS treatable with careful attention.

I'm curious, does your sibling have the livedo reticularis skin rash (it's not really a rash, but more of a visible sign of blood pooling under the skin)?

Does your sibling have other autoimmune diseases?