Can loose joints affect the heart or vice versa?

Usually not: Common double-jointedness is unrelated to heart disease, but some folks with extreme ehlers-danlos (type iv) can expect vessels to rupture when they are young adults, and Marfan syndrome people are risk of the aorta tearing. This is probably what you've heard about. Talk with your physician and perhaps someone expert on genetic disease. Glad you're pro-active; good luck.