寫英文信如修女

You are right!! But just in case, there is more than one person 姓 Chan in the organization. I suggest you to put Sister (First name) (Last name).

On the letter, I would put:

Date

Organization name
Address

Dear Sister (First name) (Last name):

XXXXXXXXX (Your letter details)

On the envelop, I would put:

Organization name
Address

Attention: Sister (First name) (Last name).

Hope it will help you!!

Sister CHAN

因為修女既英文=Sister

Organisation Name

Address

Dear sister