First off, don't do anything with the
wallet.dat files that you found on that computer (you likely did already, but maybe you made backup copies first), only work with a copy of a copy. Goal is: never change the original data in case you have to revert something.
Original
wallet.dat: don't open, only make a master copy on a different storage device. Once you have the master copy, don't touch the original anymore.
The master copy serves now as source for work copies. It's also not opened in a wallet but only copied to work copies.
Work copy: you can do with it whatever you want and what's needed. If you screw up, ditch the work copy and create a new work copy from the master copy.
I hope you get the gist.
Use a safe environment to copy the files and work on the work copies. Your daily computer is not a suitable safe environment, commonly.
You said your brother's computer is about 15 years old, but that doesn't say anything about how old or last modified (by your brother!) the original
wallet.dat files are. It's possible that you already destroyed this details if you have "worked" on the original files and not on copies of them.
Why I mention this? IIRC, wallet encryption was introduced with in the standard Bitcoin client software with version 0.4.0, released around November 2011. Depending on when those files were last updated by your brother, they could be unencrypted or encrypted.
If those wallet files actually have private keys that control existing coins AND are not encrypted than as soon as those files leak or are stolen by malware, the coins are gone (with a weak and short encryption password for the wallet, they're likely gone, too).
As long as you don't know if those wallet files "have spendable bitcoins", you should be very careful to whom you give them regarding this statement of yours:
The computer is probably around 15 years old, so I don't think it's too useful anymore. Do you think reaching out to Geek Squad or something could help?
How do I actually enter the password into the file though?
If wallet encryption is active, Bitcoin Core only asks for the encryption passphrase when you need the decrypted private keys, like when you want to spend any coins controlled by that wallet. To open the wallet file, those private keys aren't needed and therefore Bitcoin Core doesn't ask for an encryption passphrase.
If I were in your shoes, disregarding your "special" family situation, this is what I would do (with the knowledge I have, of course); I will keep it very brief, so ask for more details if you need them; there're multiple knowledgable users here to provide more in-depth details than me now in this outline:
- setup a clean computer, preferably with a fresh Linux OS, but a fresh Windows should be fine, too
- for a pruned node any storage size equal and up from 128GB is fine; a non-pruned node will barely fit on 1TB devices (including OS), better use 2TB; you don't want to run out of space at the end of syncing the node -
-- don't install anything besides a properly verified Bitcoin Core executable --
(side note: the most recent version doesn't open legacy BDB style wallets anymore, it can only migrate those to newer descriptor style wallet type and this needs additional steps // IIRC, Bitcoin Core 29.x was the last to still open BDB (old) legacy wallets) - copy copies of all distinct wallet.dat files or their folders if they originally reside in some parent ...\wallets\ folder
- start Bitcoin Core in offline mode first (not sure if bitcoin-qt -networkactive=0 will do for this, haven't tried for a while)
- at first start, I think, you're asked to setup if your node is a pruned one or not; decide according to your storage space
- import all wallet files, giving each a distinct name to separate them
- close your offline Bitcoin Core
- re-open Bitcoin Core now in online mode
it should re-open all previously opened wallets and start syncing - let if sync until finished, it will take some days depending on how much RAM your computer has, speed of internet and your storage device; you don't need to run it 24/7 but always close it via menu File>Exit to not corrupt anything
- when blockchain and wallets have been synced, you can select every wallet and see if there are spendable coins
If there's a non-zero balance in any of the wallets, then you can deal with the dilemma of your family situation (ownership of the coins).
Post edit:
The true balance of a Bitcoin Core wallet is only valid when the wallet is fully synced. Don't be fooled by anything else that's displayed while the wallet is syncing.
