Electrum will always generate a random seed phrase. You cannot pick your own seed phrase (although with Electrum you can use a different word list, although again, I would not recommend this). What you are describing is picking a passphrase, also known as a seed extension. The combination of your completely random seed phrase and your self picked passphrase will generate a wallet. Although you should strive to use long and random passphrases, using your family members' names here is less critical since you still have the security of your seed phrase to fall back on. So using family members' names for a passphrase is unlikely to lead to your coins being stolen in isolation, but pretty much defeats the point of using a passphrase if you are going to use one which is so weak and easily guessable.