There is a disconnect between wanting to be a co-sponsor for the I-864, and yet at the same time wanting to keep income details private from the primary sponsor (your daughter) and the beneficiary (your son-in-law).
As others have mentioned, consular officers can and often do ask for the sponsor's income information during the interview.
And at any point, your daughter and son-in-law have the right to request a FOIA (Freedom of Information Act) which will include the I-864 and all supporting documents, including your income information.
If you are going to go through with this, you will have to accept that they will eventually find out your income information. If this is something you really don't want to disclose, your daughter can find another willing sponsor, or eventually earn enough on her own to sponsor her husband.