I have read somewhere that knowing one or more private keys of the same seed may lead to the discovery of all the private keys.
I'm not sure exactly what this information is, but it's something close to that.
The case was related to the sale of one of your private keys (hardforks days) and keep using the wallet seed to generate other addresses/pvtkeys
You are almost there.
Having any number of child private keys does
not let you calculate any sibling private keys, as you cannot move up the derivation path and reverse a the HMAC-SHA512 function.
However, if you have revealed the master public key from your wallet (which you might have done when importing a watch only wallet, generating additional addresses, signing up for some service which requires new addresses, and so on), then the combination of the master public key plus a single child private key does indeed allow you to reverse engineer the master private key and then from there generate all the child private keys in the wallet.