"Her brother eats lemons."

Translation: उसका भाई नींबू खाता है।

[ninja_aldrin]

Shouldn't it be uski Bhai instead of uska?

[Zesul]

In any "X का Y" (Y of X), का represents the gender of Y (the possessed), not of X (the possessor). So it works a bit differently from English his and her.

[AndrNordin1]

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

[Zesul]

Just to add to what zeebo7 said, the verb is 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.

[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.

[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.

[cheenz2002]

Where do we use uska, unka, uske, unhe?

[voxtur__7]

उसका is used as "his/her" but when the possessed object is masculine. You see, in English, the gender of possessive adjectives depends on the thing possessing, rather than the possession itself.
उनका is used where the possessed object his masculine and the person possessing it needs to be treated formally.
उसके is used when the items possessed are plural and the person possessing them can be treated informally, just like in उसका
उन्हें is not a possessive adjective. It is equivalent to "them" in English i.e. third person plural in the accusative case.