The error in the sentence "Either Sana or her sisters is coming" is found in the subject-verb agreement, specifically with "sisters is." When two subjects are joined by correlative conjunctions such as "either...or" or "neither...nor," the verb must agree with the subject that is closest to it.
In this particular sentence, the two subjects are "Sana" (which is singular) and "her sisters" (which is plural). Since "her sisters" is the subject immediately preceding the verb, and it is plural, the verb must also be plural. Therefore, "is" should be corrected to "are."
The grammatically correct sentence should be: "Either Sana or her sisters are coming." This rule, often referred to as the rule of proximity, is fundamental for ensuring proper subject-verb agreement in complex sentences and maintaining clarity and correctness in written and spoken English.