2.... Is it the same in russian?
Yes. Omitting words known form context is very widespreaded practice in russian.
It can be just:
"Он показал на кольцо и пояснил:
- Сестры."
Especially if he says about something "Это - моего брата" before.
But single adjective is less depended from context here:
"- Сестринское."