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

Not much. She probably has a viral conjunctivitis. This is infectious and is transmitted by contact, usually after she might touch her eyes, then perhaps open a door leaving the virus on the handle that you might innocently open 15 minutes later. Then it can spread if you in fact touch your eyes. She should use cautionj in her environment and you the same.
What type. Conjunctivitis is a general term that there's something wrong with the conjunctiva (surface of your eyeball and lids, not including the cornea). There are several basic types, allergic, viral and bacterical. Bacterial is treated with antibiotics, viral is usually treated with artificial tears (sometimes mild antibiotics can be used), and allergic can be treated with antihistamines.