Nothing gets erased from census records.
I would have the assumption that the first wife may have mved back in with her parents...look for them in 1900 and future records, You might fine the 1st wife and/or her children as members of the parents household.

Look to her family, parents and siblings' census records.

Thanks Monica. I've done all that, but unfortunately there is no trace of her family, parents, or siblings at all. I do know her mother's name because she was living with them in 1900. Sadly, a lot of records such as those of her parents or her birth may have been destroyed in the Chicago fire of 1873.

Any chance that they died?

sushipup, I should have clarified that. They didn't die--because this was a scandal at the time in the family. My grandfather married my pious Catholic grandmother without revealing that he had a previous wife and family still living. It was only after his death that she found out.

Marita, if you have anything on the church were they were married, the church might have additional records.

There will be no Social Security records because Social Security did not exist yet in 1900.

You do have some data: the first marriage and the birth of the first 2 daughters. Does the marriage record contain the bride's maiden name and the names of her parents? If so, gather as much information as you can on these persons:

1st wife with married and maiden names.
the 1st 2 daughters.
Her parents.

If the 1st bride had siblings, check these out for additions to their households. Sometimes, children from a failed marriage were placed with and Aunt or Uncle. Also, check out siblings of the grandparents.

If you find nothing, look for death indicies in the area where they lived. If nothing, then run the indicies of local newspapers.

If the bride's parents had siblings, check these out for additions to their households. Sometimes, children from a broken marriage were placed with an great Aunt or Uncle instead of the grandparents. Also, check out the siblings of the bride.

If there was a divorce, there should be a record of this at the court house. If there is no record, the first wife could have died. Look up listings of major disease outbreaks in the US. Did one of these plagues hit at a time that could have wiped out the mother and her daughters?

There is a possibility that the mother left her husband and spirited her children away. She went into hiding and her record might be cut at this point.

Try the link I have attached below. It is for USGenWeb. Then click on the state and county they were known to be in last. You might find some information on them there, there will be various links you can click on and look thru information. Also, for most states and counties there are volunteers that will do look up's for you and their contact information will be there. Also there might be people that you can pay to do look up's. I have done this and I got a ton of information for not a lot of money, I think it was about 70.00... she found newspaper articles, obituaries, court records etc..a lot of information, well worth the money. I would also look on FindAGrave to see if you can find her parents, it might be interesting to see if you can find who is buried beside them. FindAGrave also has message boards you can read and post on, lots of people there to help also. Also, go to Google Books and put your grandfather's name and the first wife and kid's name in, they might be listed in book, if so it will pull it up. Good luck with you search..

Here is a link that might be useful: USGenWeb