"Her brother eats lemons."

Translation: उसका भाई नींबू खाता है।

[AndrNordin1]

Doesn't this translate to "Her brother eats a lemon" since the verb is not pluralised?

[zeebo7]

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.

[Zesul]

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.

[RightyKRajesh]

Why उन्हें भाई नींबू खाता है is wrong?

[Zesul]

Because उन्हें means "him/her/them". उसका is "his/her/its/their".

When "her" is used as a possessive word (as in her brother), it's translated as उसका.