From a combination of Thomas William Buckle's WW1 records, and baptism/marriage records in Leeds, George had a half-sister:
Thomas remarried in 1909 to Harriet Collins. They had a daughter, Eveline Elizabeth, born in 1910, and they all appear together in the 1911.
The WW1 service records record Eveline's birth date as 05 May 1910 and George Percival's birth date as 10 Dec 1907.
I didn't see any other half siblings in the baptisms (on a very quick search), but I have not searched the GRO for others.