1
I only have one brother.
Sign 'I', then perform the sweeping twist of the '1' handshape for 'only', bringing your palm to face you. Follow this with the signs for 'have', 'one', and 'brother'. The crisp wrist rotation helps emphasize the limitation.
2
She is an only child.
Point to the person to establish 'she', use the twisting '1' handshape for 'only', and then sign 'child'. The sign for 'only' here perfectly conveys the idea of being a single, solitary individual.
3
We only need five more minutes.
Sign 'we', then execute the 'only' sign by rotating your palm inward toward your body. Finish with 'need', 'five', 'more', and 'minutes'. Keep the movement deliberate to highlight the strict limit of time.