West Yorkshire Police have been conducting "extensive enquiries" to find Bernadett Berki, aged 12 and Szimonetta Berki, 11, after they were reported missing on Tuesday March 6.
It is thought the sisters could be with their three-year-old niece, Leonetta Bogdan.
The children lived in the Chapel Fold area of Batley, but have extensive links and extended family across Huddersfield and possibly to the Leicester area. All three are known to have strong links to the Hungarian community.
Dete Chief Insp Fiona Gaffney of Kirklees District Police, said: “We have been carrying out extensive enquiries to trace all three children since they were reported missing including press appeals. Most of the work has been behind the scenes, however, as we strongly believe the three girls are with family members. We do not believe any third party is involved.
“Work has included enquiries and working with partner agencies including those in Hungary.
“We are becoming increasingly concerned for their welfare as we have been unable to find them and are now calling on members of the public, particularly members of the Hungarian community, to come forward and help us find these girls as we need to locate them and ensure they are okay.”
Anyone with any information is asked to contact police via 101 quoting log number 1107 of 6 March or anonymously via Crimestoppers on 0800 555111.