The I-551 stamp has an A-number in it.
The I-551 stamp itself is a proof of being admitted as a permanent resident.
So you were in a better shape because you knew your A-number, had a copy of I-551 and you had a valid driver's license (Although I am still not clear how exactly you got one. You might have been able to get a DL without showing a green card before 9/11 but that DL would certainly have expired by the time you filed your I-90.)
In the case of your brother he does not know his A-number, does not have a copy of an old green card, does not have a copy of I-551, does not have a driver's license or a copy of it, does not have a passport, does not have a social security card or a copy of it. In fact, at the moment there is nothing, other than your father's say so, to confirm that your brother ever had a green card.
Now regarding whatever the burglar took. Apart from the actual green cards, there must have been some paperwork that your father filed in order to get those green cards: I-485 applications for you and your brother. Your father, at least at some point, should have had I-485 receipt notices and then I-485 approval notices related to your green cards. You should ask your father if he kept copies of any of that paperwork.
If he did not, as I said earlier, I suggest that he file a FOIA/PA request with USCIS and request a complete copy of his own A-file. Your brother's A-number might be in there.