You haven't told us enough for us to know where the photos are stored.
They could be
- In a Photos library that is local to the MacBook – which you could only access by gaining access to the MacBook and/or its backups
- In a Photos library that was synchronized with iCloud Photos – in which case, if you can gain access to his Apple Account, you might be able to retrieve the photos from iCloud (by logging into iCloud).
- Stored somewhere else on his MacBook (e.g., because he was using a program like Adobe Lightroom Classic, in place of Photos).
I would suggest that you have the Executor or Administrator of your brother's estate contact Apple, to see whether it is possible to gain access to his Apple Account. Apple will want to see proper legal documentation before helping you get into someone else's account – so the fact that you have waited "at least 3 years" (which suggests that the estate may be closed and that technically, there is no longer an Executor or Administrator) may be an issue.
How to request access to a deceased family member’s Apple Account - Apple Support
if you can gain access to your brother's Apple Account, it may be possible to use his Apple ID and password to reset his local login password without having to erase the Mac in the process.
If you forgot your Mac login password - Apple Support
