My sister has developed placenta accreta. What is it exactly?

Placenta accreta. A placenta accreta develops when the placenta invades or grows into the muscular layer of the uterus. With invasion of the muscle, detachment of the placenta after birth of the infant cannot occur. Retention of the placenta can lead to a postpartum hemorrhage. To remove the placenta, the uterus is usually removed after delivery of the infant.
Serious complication. I am sure you mean that your sister has suspected placenta accreta, as this a post hoc diagnosis only possible after the uterus and placenta have been removed en bloc and examined under the pathologist's microscope. This is potentially life-threatening diagnosis that should be managed proactively by experienced doctors and surgeons in large/tertiary care centers with large blood banks and icus.