My husband had a chest x-ray in 2015 and an aortic aneurysm was picked up just by chance at age 57. He had no symptoms, it was 4.7. We too were in shock. We had a consultation with a surgeon in New York City who only operates on aneurysms. He said although 4.7 seems so close to 5 cm. (at which they do operate) it's not.
He was very reassuring my husband does not have to walk on eggshells, only not to lift 100 pounds or more over his head. 11 years later, he is fine. It is measured annually and depending on the angle of a CT Scan or MRI the measurement can slightly changed. It went down, hovering around 4.1 to 4.6. His was most likely caused by uncontrolled B/P which is under control now. He also has severe coronary artery disease but his numbers are controlled with medications. Since you have a family history of early cardiovascular disease it "seems" like it's the culprit but only a cardiologist can tell you this. Aneurysms/cardiovascular disease can be genetic. With your cardiac profile, I am assuming you are now under the care of a cardiologist...not your GP to treat you for cardiovascular disease. Easy to say, but try to keep your stress level under control.
Sounds like you are facing major life changes...but no doubt you will do follow all your health care/cardiac recommendations and get through this!
All the very best to you.