Is it possible to determine BIP39 keys are related Is it possible to determine if public or private keys generated with a single same BIP39 phrase are related to one another?
BIP 39 is only a mnemonic standard, it is BIP 32 which describes how the derivation actually works.

Unless you have the master public key or the master private key, no, you cannot know whether two keys were derived from the same BIP 32 master private-public keypair.