When an adjective comes after sein, werden or bleiben, it takes no ending at all - the same word for a man, a woman, a thing or a group: er ist nett, sie ist nett, sie sind nett. Endings only appear when the adjective stands in front of a noun.
Examples
- Mein Bruder ist sehr nett.
- My brother is very nice.
- Meine Schwester ist auch nett.
- My sister is nice too.
- Die Kinder sind heute ruhig.
- The children are quiet today.
The mistake to avoid
Do not borrow the endings you see elsewhere: "sie ist nette" is wrong.
Where this fits in
Family & people, German A1: Talk about your family, say what people are like and how old they are, and introduce your friends.