Marcela Menyhartova, aged 13, and Marcel Menyhart, 12, from Sheffield were last seen at Ice Sheffield in Attercliffe, at 8pm yesterday.
They have not been heard from since.
Marcela is white, 5ft 3ins tall, of thin build, with long brown hair with highlights worn in a bun. She was wearing black trousers and a black jacket with fur around the hood.
Marcel is white, with short dark blonde hair. He was wearing a red hooded jumper and Adidas tracksuit bottoms.
Anyone with information should call South Yorkshire Police on 101, quoting incident number 137 of January 20, 2018.