Older siblings can make it easier for kuds to keep believing for far too long 
I've got 2 kids 24 months apart, then a 4 year gap. My older 2 were essentially a hive mind until the oldest was about 10, joined at the hip and shared everything, including thoughts, knowledge and wisdom
so yes, dc2 knew about father Christmas pretty much the instant dc1 did (dc2 was 4.5 and dc1 6.5), because they essentially thought things through together
They were both somewhat relieved as neither had liked the idea of a strange man in the house while everyone was asleep, and dc1 has asked us to stay up and ask him to leave the presents in the garage 
However the older 2 tend to baby dc3, and dc1 has an incredible story telling imagination... So every time dc3 has expressed doubts, dc1 has spun stories to make him believe more than ever (appkys to the tooth fairy too) where I was intending to let the truth dawn naturally.
Dc3 is nearly 8. I need to have a word with dc1 actually!
I don't think theres any need for a big myth busting reveal, but I do think it's best to let doubts creep in naturally without deliberately quashing them, and let them realise on their own and absolutely wrong to lie when asked outright by a kid who's worked it out.