Learn Portuguese (Portugal): As raparigas são gémeas.

We use são (from the verb ser) because being a twin is a permanent characteristic and a core part of their identity. You use ser to describe who someone fundamentally is, whereas estar is used for temporary states or locations.

Does gémeas change if we are talking about boys?

Yes! The word gémeas is feminine plural because it agrees with the noun it describes, as raparigas. If you were talking about boys, it would change to the masculine plural: os rapazes são gémeos. If the twins are one boy and one girl, you also use the masculine plural gémeos.

What does the accent on gémeas tell us about pronunciation?

The acute accent tells you two things: first, you must put the vocal stress on that specific syllable when speaking. Second, the e makes an open vowel sound, similar to the 'e' in the English word 'pet'.

I often see the word meninas for girls. Is raparigas different?

In Portugal, raparigas is the standard, everyday word for girls, teenagers, and young women. The word meninas is usually just for little girls. Keep in mind that while rapariga is perfectly normal and common in European Portuguese, it has a negative meaning in Brazilian Portuguese!