My sister was diagnosed with esophageal cyst. What do I need to know?

Resect. 75% of oesophageal cysts will become symptomatic, therefore , resection is advised as soon as diagnosed. See your board certified thoracic surgeon.
Rare. But often seen. They can become large causing difficulties swallowing, or they can become infected. The generally do not become cancerous. They should be surgically removed, and this can often be done thoracoscopically or robotically. Talk to a board certified thoracic surgeon.
Esophageal cyst. Esophageal cysts need to be monitored closely. Surgery to excise it may be necessary.