The student I referred to above who tried to transfer from Choate to Andover and was refused, had an older sib at Andover at the time. Younger sib ended up at Exeter, so family had PA, PEA and Choate students. At least a couple were attending at the same time. It seems like a lot for one family to keep track of with drop offs and pick ups. Glad my two went to the same BS, which is why I do think siblings have a slightly better chance.
At Choate I think we got a letter for the second student (sib) which stated the admit rate was 40% so that meant that still the majority of siblings will NOT get admitted.