Found this on another forum. It looks as if David was posting there as well, but his posts were removed or unreadable for guests. It's some information.
John only met David once, and I read that when Alfred and Pauline brought David over to see John, John threw them out of his house! Not sure which book that is from tho...but I guess John was aware that he had another half-brother, Robin, considering the fact that both Alfred and John did keep in touch until Alfred's death.
David and Robin have been included in Julian's website of his Lennon family, Lennon.net, so that must have a lot more information that can help you.
As for Victoria, her other name was Ingrid Pederson who worked (or works) as a nurse in England. She came out as John's long lost sister some years back after her adopted mother passed away. She had learned her connection to John in 1966 but kept it a secret on her adoptive parents' advice. One of John's aunts told John about his half sister and he tried to look for her, but had no luck. Last I heard, Stanley Parkes (John's cousin), Julia, Julian, Cynthia, and Yoko have welcomed her into the family. You can google her for more information, some links have been brought up on the subject (I typed in John Lennon's long lost sister)
His both Brothers named David Henry born February 26th 1969 and Robin Francis Lennon born October 22th 1973 in Brighton,