Both me (1)/ and my sister (2)/ went to a boarding school. (3)/ No error (4)

Answer

1

Explanation

I will replace me because -
me is the Objective Case of I. In the place of a subject, we always use
Nominative Case
Look at the examples given below:
Sheela and me are doing our homework together.
Sheela and I are doing our homework together
Hence, Both I is the right usage.