BIP32 child addresses - anyway to corelate them?
No, from the outside all addresses look completely unrelated, there is no way to work out if two addresses have any relationship to each other, including if one is the parent of the other or sibling of the other, without the chaincode as well. That's because all derivations involve hashing, and hashing is designed to have an apparently randomly distributed output, regardless of the relationship between two inputs to the hash function the outputs should appear unrelated.