Nyah James, 14, was found dead at her home in Blaenymaes, Swansea, on 6 February.
Jordan Clements, 20, told one girl she would be "strung up with the dead cows in the butchers".
He admitted two counts of harassment and two of sending communications of an indecent or offensive nature.
Swansea Magistrates' Court heard how Clements told police he sent the messages when angry and upset and wanted to make the girls feel the way his sister had felt.
His victims - who cannot be identified  - felt frightened and anxious by the messages, the court heard.
The case was adjourned for reports.
An upset brother sent abusive messages to four schoolgirls he thought had bullied his sister before she died, a court heard.