Just to add to what zeebo7 said, the verb is not pluralised not because of the number of lemons but the number of the subject (her brother).

उसका भाई नींबू खाता है। - His/her brother eats lemons/a lemon.
उसके भाई नींबू खाते हैं। - His/her brothers eat lemons/a lemon.

नींबू doesn't change when made plural (like 'fish' in English), and the verb conjugates with भाई. So here it could be interpreted either way really (एक is sometimes omitted). I think it would be largely context based.