Breakdown of Они внешне похожи на сестёр, но их поступки разные.
сестра
sister
они
they
на
on, at / by (transport) / to (a place) / to (in answer to)
их
their
Questions & Answers about Они внешне похожи на сестёр, но их поступки разные.
Why is сестёр in this specific case?
The short adjective похожи requires the preposition на followed by the Accusative case. Because sisters are animate (living beings), the Accusative plural form matches the Genitive plural, which is сестёр.
Could we just say Они похожи на сестёр without внешне?
Yes, you can. похожи на simply means similar to. Adding the adverb внешне (externally) specifies that they physically look alike. The second half of the sentence then contrasts this physical similarity with their behavior.
Why is разные a long-form adjective when похожи is a short-form one?
The word похожий is almost always used in its short form (похожи here) when it acts as the main action or predicate of the sentence. The adjective разный, however, usually stays in its long form (разные) even in this position.
What is the singular dictionary form of поступки?
The singular form is поступок (an action or deed). Notice that the о in the last syllable is a fleeting vowel, meaning it drops out when we add the plural ending -и to make поступки.