Learn Spanish (Latin America): Ellos también son hermanos.

Ellos is the masculine plural word for "they." It is used because the sentence talks about hermanos (brothers). If the sentence was about a group of women, you would use ellas instead.

Where does the word son come from?

Son is the plural form of the verb ser (to be) that goes with ellos, ellas, or ustedes. Because the subject is ellos (they), we use son to mean "are."

Could Ellos también son hermanos also mean "They are also siblings"?

Yes! In Spanish, the masculine plural form is used for mixed-gender groups. So hermanos can mean either a group of just brothers, or a mixed group of brothers and sisters (siblings). If the group was entirely female, you would use the feminine plural and say Ellas también son hermanas.

Can I put también at the end of the sentence, like we do with "too" in English?

You can, but it is very common in Spanish to place también right before the verb. Ellos son hermanos también is perfectly understood, but Ellos también son hermanos is the most natural word order.