Sita or her brothers . . . . . . . . to be blamed.

The verb to be placed in the statement should agree with the subject of the sentence.
In the given sentence, the subject is 'Sita or her brothers'.
Here, due to the usage of 'or', only one person is performing the action depicted by the verb.
The verb placed in the sentence will be in accordance with the word immediately before the verb.
Here, brothers is a plural noun, hence the plural verb is placed.
Out of the given options, only 'are' is the plural verb.