Alternative subjects are connected by or or nor:

When both subjects are plural, as they are in the example above, or when both are singular, there’s little cause for confusion. But sometimes the alternative subjects are different in number; in that case, the verb should match the subject that’s closer to it in the sentence.

I don’t know whether my brothers or my mother have more friends.

If the corrected version seems awkward, you may choose to reorder the subjects. Here, the alternative subject brothers is closer to the verb, so the verb should be have:

I don’t know whether my mother or my more friends.