My cousin was told she miscarried one of her twins at 5wks, but the other survived and is growing. how is this possible with all the bleeding she had?

NOt that uncommon: It is very possible to "lose" a twin and continue with a healthy pregnancy. In my practice of 40 years this has happened multiple times!

Not unheard of.: Most recent data suggest that many of us started our in utero lives with a co-twin that never survived to viability, or even detection by ultrasound. Losing a dichorionic twin and continuing the pregnancy with the survivor is actually a well-known outcome in multifetal pregnancies. The earlier the fetal loss the less severe the consequences to the survivor, because these insults are all/nothing.