Check the case number on your brother's cancelled check. If you try the USCIS website with 1-4 digits above that number, does it come up with the I-130 and I-485 receipts online? If so, and if the number is different from your mother's case #, then you're probably fine.
Per the August filer thread, Chicago lockbox is slow and erratic lately. So what you're experiencing isn't unusual.
-ML
K1 -> I-485 -> I-751 -> N-400
Now filing I-130/I-485 for parent...
DO: Santa Ana, CA
5/13/08 - Parent I-130/I-485/I-765/I-131 mailed to Chicago Lockbox
5/22/08 - checks cashed, receipt numbers from Missouri MSC (NBC)
5/25/08 - Notice dates for all notices
5/29/08 - LUD I-130/I-485/I-765/I-131 (received and pending at NBC)
6/07/08 - Fingerprint appointment letter received
6/12/08 - Fingerprint appointment date
6/12/08 - LUD I-485/I-765 (no change, still received and pending at NBC)
7/29/08 - LUD I-485 - RFE sent out, case on hold
7/30/08 - LUD I-485 (no change, still on hold for RFE)
8/04/08 - RFE received, request for tax return, 87 days to respond
8/05/08 - Sent RFE response to NBC (regular mail)
8/14/08 - LUD I-485 (RFE received, case processing has resumed)
8/15/08 - LUD I-485 (no change, still case processing resumed)
8/18/08 - LUD I-485 (no change, still case processing resumed)
8/18/08 - LUD I-765 - EAD approved / card production ordered
8/18/08 - LUD I-131 - AP approved / approval notice sent
8/19/08 - LUD I-131 (no change, still approval notice sent)
8/26/08 - AP received, multi entry valid for 1 year from 8/18/08
8/29/08 - EAD received
8/30/08 - Case transfer to CSC (letter received, no web update)
9/03/08 - LUD I-485/I-130 (case pending at CSC)
9/04/08 - LUD I-485/I-130 (no change, still case pending at CSC)