Sibling rivalry

So just before midnight on Valentine's Day we get toned out for a stabbing assault. We arrive to find private ambulance medic in full blown panic. But to her credit she has already loaded the patient onto the gurney and we are on our way shortly. After all, we are the tow truck, not the mechanic. So the transport goes well and we deliver our unconscious female patient alive. Strangely enough the female patient was stabbed by her sister. In front of both of their kids ages 4-6 years old. I just wonder what the argument could have been to make someone stab their sister several times.