"Her younger sister looks upset."

Translation: 她的妹妹看起来很难过。

[LexyCarrie]

Should be 她的妹妹看起来难过 cause there's no 很 in the question

[AbunPang]

Usually when an adjective serves as a verb, you need an adverb of degree like 很 (or 真, 非常 etc). You can't leave it bare. 很 is the weakest of these adverbs, so it acts as a placeholder when you don't want to express a particularly high degree. Only when 很 is emphasised, it becomes equivalent to English "very".

[Mokuhazush]

This 很 doesn't mean "very", I think it's the same kind of 很 as in 今天天气很冷 = it is cold today, where 很 basically means "is" (很 is used for adjectives and 是 is used for nouns)