It's a myth, all the Birmingham grammars are specifically prohibited from giving priority to siblings by the Admissions Code:
Para 2.16: "In setting oversubscription criteria admission authorities must not
:...m) in the case of designated grammar schools that rank all children according to a predetermined pass mark and allocate places to those who score highest, give priority to siblings of current or former pupils;"
And para 2.74: "Methods of allocating places for oversubscribed grammar schools vary. Some admission authorities allocate available places in rank order of performance in the entrance test; admission authorities for these schools must not
give priority to siblings (see paragraph 2.16(m))."
And para 2.95: "Admission authorities must not
adjust the score achieved by any child in a test in order to take account of oversubscription criteria, such as having a sibling at the school."